only zx3 doors will be the same size...iirc, 00-07 should be a direct bolt in (05-07 will have different, unmatching trim pieces, swap yours over)
any zx3 seats from 00-07 will swap with the best and sportiest coming from the SVTs. if your seats have airbags, then you may be limited to the same year/model to ensure those are plug and play. If no airbags in the seat, then they all should work, just reuse your seat belts. Other model seats (from 4 door focus) would bolt in but zx3 needs the fold/slide forward that are only on other zx3 seats.
Cheap and light usually are mutually exclusive when it comes to wheels, unless you want cheap wheels that are garbage. Decent wheels can be found in the buy/sell trade section on this site, lots of good things there. Popular wheels are those from the 17" SVT wheels as well as 16" alloy from other years. Any oem focus wheel from 00-11 will work. Wheels need to be 4x108 and should have an offset from 40-50. 15/16" wheels will have slightly cheaper rubber and 15"s will provide the most sidewall on a tire, might be better for rough roads from winter driving.
for new, Sparco makes some alright ones that wont break the bank and are still light. The assetto gara is pretty popular and is lighter. Head over to www.tirerack.com
and type in your car, they will list the wheels that will fit (ignore the photos that show 5 lugs, just a stock photo)