Thermapens are nice, but not nearly $100 nice. I really like the CDN digital. $16 and gives a reading in about 6-8 sec. Very reliable and easy to calibrate....though I have never needed to re-do after the inital set.
pH meter? There's just no cheap way around that, so that's where I'd put (have put) my money. If you're re-brewing the same recipes, you won't need it that often, but it's nice to know for certain exactly where you are while you're nailing down that recipe. Martin's worksheet will definately get you in the ballpark most of the time, but that pH meter gives you complete accuracy. Be prepared for the ongoing costs of calibration solutions. Unlike the thermometer, there's a little maintenance with the meter too.