The clutch on the Yokomo is fine, it's the two speed that some people have had problems with. The newer two speed on the R and PRO seem to hold up very well, though. I am converting to the Serpent two speed and Centax, since I can adjust a Centax clutch and the two speed is a shoe type two speed.
As for what parts to get, what are you planning to use the car for? For tight tracks, stick with the PRO and front and rear diffs with rubber tires. For large sweeping tracks, buy the R and use foams with the front one way and rear solid axle (all stock on the R). I bought an R, but the PRO would have been a better fit for me. The PRO wasn't available at the time, so the R was my best choice.
The aluminum side pulleys are only for use with foams with regular sized front foams and inch up rear foams, as the aluminum side pulleys overdrive the front wheels quite a bit to make up for the smaller diameter front tires.
As for hop-ups, I would HIGHLY recommend the aluminum clamp hex hubs and the aluminum rear bulk head brace (stock on the PRO).
If you buy the PRO, and you're interested in the solid axle, drop me a line, I would be interested in buying a stock differential off of you, and perhaps working a trade for a solid axle.
If you're curious about my setup, I bought the R, and I have:
1 stock planetary diff with Yokomo hard diff lube
plastic side pulleys (for use with rubber, and the stock aluminum ones for use with foam)
200mm rear to use with rubber (the R comes with a 190mm rear to accomdate 32mm wide foams)
Penguin upper radio plate
Penguin graphite brake disk
Penguin front bumper brace
rear aluminum shock tower
front aluminum shock tower brace (R comes with the rear aluminum shock tower brace)
aluminum rear bulkhead brace
aluminum clamp hex hubs
aluminum radio plate supports (the R comes with the aluminum radio plate posts)
Aluminum layshaft mount
Serpent Centax clutch
Serpent 2 speed tranny
Smitty Serpent conversion shaft
Smitty Ti hinge pins
Smitty Ti front layshaft mount
Collari XS12RTS 5 port turbo engine
Yokomo tuned pipe by Meilke (or however you spell it)
I also added small springs in between the brake pads to reduce brake drag.