I replaced my hot water heater because I had to. The old one stpped working the week after I closed on the house! I had to replace my furnace a few months later because, unbeknownst to me and apparently the useless house inspector, the old furnace was improperly installed so that CO could easily be vented into the house through the ducts. Jeepers! So I had to replace both, along with a big chunk of plumbing and upgrade the electrical to handle the new HVAC. All this is to say that one thing can lead to another, so you might want to budget for the worst.

(then my roof failed and I had to get a new roof...but that's another story)