Save your money and buy a copy of Berlioz/Strauss Treatise on Instrumentation. It’s my bible for orchestration.

You would need years of training before owning a violin would give you any idea of what is possible… I would recommend studying scores instead. It’s like saying I’ll just buy a fighter jet to know what kind of tricks they are able to do. Without the training it really doesn’t teach you anything.

That's a reasonable approach tbh if you can afford it

Key things to know:

• there are 4 strings, typically tuned from lowest to highest as G D A E

• there are 4 fingers, typically placed to cover up to a tritone with a maximum gap of a minor 3rd on one string (if you're very good at writing you can demand more, but that's not for now) – the pointer is the first finger, the pinky is the fourth

• the first finger can reliably be placed anywhere on the neck up to maybe a 12th/octave + 5th above the open string. Changing where the hand is is called "shifting" and can be done reasonably quickly, but gets challenging if required very quickly and very often

• the bow is very light and agile, and is capable of playing sustained for up to maybe 20 seconds at moderate volumes but is capable of playing very very fast for long passages

• the easiest motions are: between open strings and any note played with a finger on the same string and between two fingers on the same string. Less easy: shifting the location of the hand

• string changes can help you cover large distances quickly, but changing string very quickly and very often can be challenging

• passages which include many repeated string crossings are best when the player can keep their fingers as still as possible - design your hand shapes (no duplicating fingers across non-adjacent strings, be mindful of where fingers need to cross or be close to each other) to be comfortable and chord changes with the smoothest possible voice leading

• double stopping is the act of playing two strings at once, usually to play two different notes. Make sure that both notes can be reached on adjacent strings with the hand in the same position. For example, a double stop of an A3 and a C3 is not possible because both of those notes can only be played on the G string, and a double stop of an A3 and a C4 is not advisable because the C4 is extraordinarily challenging to reach on the D string while also stopping the A3 on the G string

• the bow is capable of an enormous range of articulation and techniques, the various levers you have to pull are speed, weight, on/off string, position in bow (heel/tip/middle/full), bow position in string, bow orientation, bow direction, all of which can have audible effects on the sound. This one can only really be understood by studying and learning the relevant vocabulary, not summarised in a comment

• for much of the midrange of the instrument, you can play notes on up to 3 strings which all have very different tone qualities - be mindful about which you're picking for your desired colour

• fingers get closer together and good tone quality gets more difficult the higher up the fingerboard you go, requiring the bow to be placed closer to the bridge – be careful, and try not to be as demanding of speed at positions > octave + 5th above the open string

• easy harmonics occur when you gently touch the string on the octave, 5th, 4th or major 3rd above the nut, which respectively sound a cool pitch an octave, octave + 5th, 2 octaves, and 2 octaves + major 3rd above the nut. The smaller stretches can be achieved with stopped notes (e.g. stopping an A3 on the G string with the first finger and gently resting the 3rd or 4th finger on D4 or E4 also on the G string sounds an E5 or A5 respectively). These stopped harmonics are really not very agile so unless you're Paganini try to be very deliberate in your use of them, but the open string harmonics are often really useful landmarks so feel free to use them as you like

• we have a mute that makes a cool sound, similar change to that of the sostenuto pedal

• playing high up on the G string is a beautiful sound in slow pieces

• read lots of violin music

There is no way you can understand violin just by buying one and trying it out.

Read sheat music made for violin.

Learn by studying scores.

Depends on what you mean by composing for strings. If you just need to write a string part in a symphony, you're fine as long as you stick with relatively simple lines. if you aim at writing virtuosic/solo works you do need to get a more intimate understanding.

Generally speaking, you need to be careful with lots of quick jumping around and/or quick repeated larger intervals, as they can be extremely awkward, depending how they lie in the hand and fingers. This applies also for accompaniment parts. Pianists may overlook that what is easy for them isn't so for different instruments. You can only assess what is easy or playable with practice (like the rapid broken chords/intervals in Vivaldi) and what is hard by knowing how the notes are played on the strings.

Violinists generally love longer melodic lines, for which the instrument is eminently suited. Arpeggios and scales are also easy for professionals as these are part of basic training.

I think having a cheap violin on hand isn't a bad idea imo

In order to understand what's possible or what's complicated to play, you'll need to be proficient at the instrument first (at least several years of serious study, probably more).

If that's not something you want to commit to, better options are to consult with violinists or study violin scores.

Avoid a Tchaikovsky 4. meaning writing fast notes in groups of 5 which are easy on piano but unplayable on violin. Or Wagner Valkyries. Never be Wagner in any case.

Edit: I have absolutely no experience of strings, only have played guitar once.

Is this for Solo violin parts against the piano?

There are lots of good answers so far. But what I haven't seen discussed yet is key (or numbers of sharps or flats). 

C major (no sharps or flats) is good. One sharp or flat is good - I actually find those easier to play than C major. I think most beginners will stop / be unable to play well (or want to even try) if there are more than two sharps or one flat in the key signature. 

The more intermediate players are probably good with 3 of either. 4 is doable, but not really enjoyed.

From there, as experience grows, people are better able to handle more. But no one is going to be super happy looking at 6 or 7 sharps or flats.