[Desmo Rider]

I would go with a S4RS, hands down.
The problem with a S2R or S4R is that if you own them you will be wanting for more (which is the S4RS), but you won't have that issue with the S4RS. Besides it will be a future classic and it's an amazing bike to ride.

 

[Desmo Rider]

The S4RS have a 998 Testastretta engine. You can get the S4R with the same engine but you have to get a newer one. Older models had a 996 engine.

You can upgrade the brakes etc but you will end up with an upgraded S4R. I would look for a low mileage S4RS.