The S2 is a good "starter" for 50mm work and you can add the 35f2.5 to it later. A good user S2 with a 50f1.4 should be doable within your budget and still have enough left over for a small lightmeter (Gossen's Digisix) and, with some careful watching, even the 35f2.5 (use the "full view" in the finder - close enough.)
The S3 is more modern as stated, and tends to be a bit smoother - but you would probably blow most of your budget on one body and the 50f1.4.
There were some changes to the S2 over its life - but nothing that makes one version better as a "user" than an other. Black Dial versions tend to be a bit more - but I find that they are no different than the chrome dial in function.
Check finder "clarity" and rangefinder patch - get the best you can in this aspect. Cosmetics are less important - Nikon Rf's pick up dents and scratches with alarming ease - so pay for good mechanical condition and clean glass in the lens, rather than cosmetics..
Nikon Rf's are not that expensive compared to Leica - but accessories tend to be expensive, hood's, finders etc. The good news is that there are a lot of "non Nkon branded" stuff available (Heavy Star Hoods, VC finders (35 and 28).
I find that in general, the Nikon Rf's are holding up better in long time use than my M's. Rare that rangefinders go out of whack, shutter speeds tend to stay pretty constant and they seem to survive "inactivity" better than the M's.
I have been a dedicated M-user for most of my life - but in the last 10-15 years I have started to use Nikon Rf's quite extensively - and I like them. The long focus throw makes it easy to pin-point focus, the optics are top-notch and. yes, they have a certain "cache" too.
Go for it,
Tom