I have a couple of DSR-4200s. One is a 4200C and the other is a 4200v. One word of warning, in order for a 4200 to receive any FP signals it must have been previously authorized and not had the authorization reset when it was removed from service. Most of the ones you see on ebay probably have been authorized but you probably want to make sure the receiver is not new and not still in it's original box. If it is, it may not have been authorized and as a home user you'll probably never be able to get it authorized. This is one case where "new" is not better or at least not for FTA purposes.
Also there are several different models. I know of at least 3 different 4200s. I know there is a 4200B, 4200C, and a 4200V and the last letter makes a difference in what signals these receivers can tune. I believe the 4200V can tune a wider variety of signal than the other 2. I know that my 4200v tunes signals the 4200c cannot. The 4200C doesn't seem to support any signals with a symbol rate of 29270 whereas the 4200v can tune some of those. Also none of the 4200s support combined mode or megapipe. Another thing to keep in mind is that all of the 4200 receivers are getting old and that might mean they are more prone to battery failure.
As for 4200s expect to see more of them on ebay in the future, as one of the providers of signals these receivers were most often used for seems to be moving away from DCII and toward DVB. Alot of educational institutions may be selling off their 4200s soon, if they haven't already or sending them to the dump. Bottom line if you can get one for $50 or less, it might be worth it just to play around with a bit, but I wouldn't recommend investing a great deal in one of these receivers at this point. Also don't expect to get a whole lot of new signals on one of these. Most of the signals you can get with these have the same content (or very similar) that you can get with a DVB receiver.