Well a quick check on Tire Rack shows a set of BFG 285/70/17's, at $217 per tire. For a tire with an 18" wheel, you might be paying about $100 more total... So not a huge savings, but certainly not throw away money. In this case, I would just suck it up, pay the difference and keep the same size wheel....
Unless, the new wheels were stupid cheap....maybe I would go with them. But I'm still running stock wheels with Plastidip, only because I couldn't find a set of wheels I liked, and that I could afford, and that was in the same size..
Edit: As tcarr says, generally more options for a 17" tire...but 18" tires are not obsolete, so it's your call.
Hmm..another quick search gives me this.. A set of AR708 wheels, in a 17x8.5 size, at $728 for the set, plus a set of DC Trail Country tires at $956 for a set of 4.. And obviously, you could go with any wheel or tire brand you wanted. This is a very generic search, looking for 17's and 285's... Buy you're in for about $2000 with taxes, delivery, leveling if needed, etc...