When you talk about the 100gn bullet are these factory loads? What rifle do you have? There are plenty of good bullets between the 75 - 100gn range. Are you handloading?
My longest shot on a groundhog was with the 95gn Ballistic Tip. My rifle (Win70 HVB) also likes the 80-85gn range. As a matter of fact, I'm fixing to start load development shortly on the Sierra 85gn HP. I have tried other 80-85gn bullets in this rifle and it shoots them extremely well.
Don 't forget the 90gn bullets; Nosler and Berger make fine bullets in this range.
I have not tried anything above 95gn in my rifle and I'm satisfied to say it has shot everything accurately from 65gn up to 95gn. This is my dedicated varmint rifle though. I have others in 6.5-284, 7mmRemMag and .300WinMag for targets and big game. I would not be scared to press my .243 into service for deer with the right bullet, however.
Spending your money, hum, let me think? You could buy reloading supplies and try other bullet weights, like some other members suggested, or you could buy a tighter twist barrel and stick with drop charts (what I use). Or you could just purchase the PDA/Exbal and use it for now with your .243. Then when you outgrow this rifle and/or purchase another later on you'll have the PDA/Exbal for it.
Several ways to skin this cat, you just need to fingure out if you want to start at the head or the rearend first.