first, even though i like pioneer a lot, i think there are better deals for that kind of money. my main reason is this... the 8500 is very different from the new 8600. the 8600 has auto eq'ing and auto time alignment features which were actually the only reason i even bought one of these. they are really good features but the 8500 only has a normal 3 band parametric eq and no time alignment features. in fact, the 8500 is almost exactly the same as the 8400. they shouldn't have even bothered putting out the 8500.
some options could be spending a little more and going for the p8600mp, which you could probably get price matched by sounddomain for about $340 shipped or less, which includes the things you need to install the unit. you could do that or you could get the 7600 for about $220 shipped with the things you need. this unit has basically all the features of the 8500 but no motorized flip down faceplate.
also, the guy said he'd install it but only said to get the installation kit. but i doubt he has the antenna adapter or wiring harness. he may not have known it needs an antenna adapter and he may have just planned to cut off the stock harness which you DO NOT want to do. so if you want to go through with getting the 8500, i'd go buy everything online from the same place so you get good deals when you figure in shipping. you could even have sounddomain price match stuff and save a few bucks. if you want to get it locally, walmart should have the harness (that is, if you aren't upgrading from the mp3 unit) for about $7. and the antenna adapter can be had for about $8 since best buy and circuit city are your best bets for carrying it (check the link from FocusInCali above for the details on that). last, the installation kit will probably cost $15 from best buy or circuit city. if you can go to a metra or scosche dealer, they'll probably have everything you need. as well as any chain install shop like car toys.