I don't know I'd say "don't buy" just because you may go DSLR in the future. I'm using two DS50's (old versions of the 51) with a DSLR now. I will say I wish I had more light power for wide angle shots. But they've not been useless by any means, and I am actually out there shooting whereas if I'd waited to afford DS125s I wouldn't be there yet.
If you know all you ever want to shoot is super wide angle stuff, then you know you will want more light later on. But then you'd be going absolutely crazy shooting now with a PnS (sorry, when I see POS I think Piece of ....
) and no strobe! If you like shooting more close-up macro and/or coral texture stuff, the 50 will be just fine.
Don't know where you are (in the states or otherwise) but you should be able to get a used DS51 strobehead for about $300 or a bit less. I've got an older DS50 (one I can't use with the updated iTTL in the Ikelite housing) on sale for ebay right now with the EV controller, arm, tray handle, etc. and if I get $325 for the whole setup I'll be quite happy (NOT intended as a plug - there are plenty of watchers, even with no bids yet I'm comfortable it'll sell). Cost me about $400 combined and I shot with it for a year with my little Canon A520 before taking the plunge and upgrading to my Oly E330.