1) I've never heard a person could "over power" an amp with a particular pickup. However, the type of pickup definately will be a large part of the basic tone. What type of sound do you want? Single coils will be sharper, more defined, and humbuckers will be fatter, and less defined. If you don't need to be screaming loud, 50 watts is plenty of power to leave it turned down, and get a fairly clean tone... but, I don't have any practical experience with that particular amp and others may have better input than I.
2) I really like Waldo's UGB, and, see response to #3
3) If you like the early 70s sound, I have a strat clone that I put new pups in (which I make myself) and a Stratoblaster, and I'm really happy with the tone of that one. I am partial to the single coil sound and my latest, best, favorite guitar so far has single coils with Waldo's UGB and the effects loop.
4) You could put a hard tail in if you fill the hole properly. On the previously mentioned Strat Clone, I removed the trem springs, put a wood block in to hold the bridge ridgedly in place, and used the cavity for the battery.
5) I really prefer the narrower vintage frets, you get a better, cleaner, more accurate note with them. I think it's a bit harder on the finger tips but what the hell, it all hurts for a while anyway
Check with a tech and see if you can't simply get a fret leveling/dressing to get a few more miles from your axe.
Brian




