good effect on interiors is to mix colors. maybe blue dash, door tops, and seats (or whatever color yours is) and mix in a complimentary color for the flat vinyl on the doors or carpet or console. not that any of that would actually look good, but grey and blue go together well and it makes it easier to find parts.
stay under/at a 25 inch total tire diameter (look up the tires by manufacturer to make sure the diameter is right, the printed tire sizes are approximate). as long as your suspension is good you can clear it with no lift (if you lift it, then it depends on your lift). I like to find Peugeot alloys cause they come in 14 and 15 inch sizes and have the proper lug pattern. other than that you can drill your hubs for the common 6 bolt pattern, or drill flat center 6 bolt rims to fit your pattern.
advancing the timing is more about usage than anything else. changing it just alters where your power is, not how much there is. extra advance is great for town driving, light throttle, etc. less awesome for full throttle usually, but how much depends on the motor. id leave it alone until its running very well and you have driven it enough to feel the difference.
webers are great, but not so great that I would pull a functioning hitachi for it. if your carb is running well let it be. if its not I would probably replace it rather than fix it, unless its something very simple. same as the timing, it doesn't really give you "more" power as much as it changes when the motor generates it.
pull the cat if you wish, but I would probably check your fuel and ignition systems out before going to exhaust to solve your top end problem. plugged cats just aren't as common as people think, though it can happen so worth checking into if the problem persists. I have seen mufflers plugged with fiberglass as well, and a much easier check/fix.