Sailor Steve pretty much hit the nail on head.
If I were in your shoes I'd proabably start with SH3 or 4 on the basis that they cost next to nothing now and they are more mature in terms of mods and fixes. Also going back to SH3
after Sh5 will probably hurt your eyes
If you like what you get, then pick up 5 (which isnt awful at all, its pretty decent now)
but it still needs another 2 years of TLC from modders before it can truley shine.