90K is very low mileage for a from 1988.
Realistically, what else could you drive for $500?
All used cars need some type of repair.
Intake leaks are not common for this generation of Bonneville nor does it have the crappy plastic hose connectors that crack over time.
Read the post by myfirstbonnie in this thread for typical slow leak spots
post309709.html?hilit=intake#p309709A cooling system pressure tester is the absolute best way to find a coolant leak.
It's what repair shops use to find them.
You can borrow one from some parts stores (Autozone, etc.) or buy your own (Rockauto--good prices forum discount on orders--look in Pontiac Bonneville Club Vendor's forum post for code).
http://www.rockauto.com/en/tools/coolin ... ng+kit,249The Stant and the Gates kits appear to be identical.
Your cost from Rockauto could be less than $450 including the cooling system pressure tester.