welcome to this crazy new world!
i'll try and address the issues i know.
1. speedo, plastic and seat are all the same across the whole line of t-dubs. speedo, had a trip odometer added in 2001 or '02. for plastic, there are a myriad of colors to chose from, or you can paint them too. seats are easily recovered, there are several sellers on ebay, i paid under $20 for mine. many opt for seat concepts new pad and cover, but thats closer to $150.
2. paint, do a search for many threads on paint people have used for frame and engine. your black widow color scheme is a popular one.
3. there are some engine parts that are usable off of the xt225, but not many. lzrdbrth has done a list of parts from several bikes including the ttr225 that swap.
know that the '87 is a one off year for the cdi, they have been known to be problematic, and parts are near impossible to get.
enjoy the bike and there is a wealth of info on here to make your ride better, faster, stronger.... but like steve austin, it can cost 6 million+