I don't know*** !
I don't know about service kits from eBay, but when you're in that venue you pretty much go on faith til proven or disproven.
I don't know if the internals of the 424mk2 are similar enough to 424mk3, but on gut feeling I believe they are. They may be identical.
I don't think you need to be concerned with replacing the pinch roller of a 424mk3 if you keep the original one clean and check it once in a while. It takes quite a while and environmental assault for a pinch roller to break down, however they will become dirty and slick which may cause noticeable problems. Alternately, having a Mk2 pinch roller on hand could always be kept in reserve for a rainy day. More likely than not that belts and pinch rollers will be compatible between the Mk2 and Mk3.
A pinch roller is beyond it's service life if it's hard, slick and cracked, or worse yet has flat spots or detents, but sometimes dirty and servicable pinch rollers can be revived with proper cleaning or rubber cleaning/restoring compound. A real no go is if the pinch roller develops a gooeyness and that's really fatal at that point. Rule of thumb in my mind is the pinch roller rubber should look and feel identical to that of a new car tire. If you're in doubt, go check it in person the next time you're at Walmart or other venue selling new tires.
The belt and pinch roller,... just call Tascam parts @ (323)727-4840 and find them there for proper sizing and reasonable cost. The only consideration there is they do not ship internationally.
Wow and flutter are used as a synonymous phrase. In my
nonexpert opinion I believe wowwing would be more often relative to the belt and flutter would be more relative to the pinch roller, perhaps among other more less likely secondary issues. In general, as I think there are exceptions to every rule and a host of other unknown factors.