What wears out first are the splines that go to the auger itself.   On the 99's they finally put a grease zerk there to help a little.   The flighting also wears heavily, even with hard surfacing.    The gear boxes themselves hold up fairly good if kept full of cornhead grease.   Parts from the 97-98 don't interchange with the 99 style.   The gear boxes are different, one style has 3 gears and one has 2, so the augers turn in the opposite direction.   The result is, when you upgrade you have to everything.  

The 87 should have no trouble with a 6 row head and 200 bu corn.   Worst case is you can't drive 5 mph.

I've a TR97 as well as a 99.   Bubble augers are a problem but it's better if upgraded to a TR99 auger.   My 97 has 3500 sep hours and is going strong.   I actually have to replace the whole bubble up auger before next season.   As said, parts are expensive at times but one thing I've found is a surprising amount of stuff fits from a TR85.    If you have a bone yard nearby that helps.   NH combines throw the cleanest grain sample I have ever seen and yet lose very little out the back.   Check the cross auger in the bottom of the hopper for wear, especially just before the elbow.   If the flighting is worn, it needs to be replaced and I think it's around $1200 just for the auger.

Overall, the TR's were good combines and vastly underappreciated.   I've said it for years that if the TR's were painted green and said JD on the side, every farmer would have two of them and they'd be proclaimed the greatest harvesting innovation in the last 100 years.