Just some random thoughts:

I would place my highest priority on a straight and clean airframe.
Everything else can be fixed with money. This doesn't mean a plane with
damage history is out of the question, just a caution to make sure it was
repaired properly, which may or may not be easy to determine. Just because
the logs said it was repaired properly is no guarantee of the quality of
workmanship.

Then I would look for an airplane that has as all of avionics that you want
already installed. This is where you will get the most value for your dollar
compared with trying to upgrade the avionics afterwards. It's almost like
finding money. Your old stuff won't be worth much and the new avionics plus
installation will be very costly.

I would look for mid time engines that have had no major repairs, no known
problems, normal oil usage, and good compressions. Again, this should be
the best value while providing several years worth of service and a history
of trouble free operations. Check the logs for regular oil changes.

Study the log books. Spend twice as much time looking at the logs as you do
at the airplane. Look for trends and recurring problems. Talk to the
previous owners, not just the seller, and talk to the previous mechanics.
Talk to the shop that overhauled the engine, get a parts list of what was
replaced with new or overhauled. Look for evidence of Service Bulletins
being complied with. These are not a requirement for a part 91 operator and
may be evidence of an owner that wasn't afraid to spend money on his
airplane. If you see SB's being complied with but no 100hour inspections
you can assume that it wasn't on a part 135 service, but the owner pulled
extra cash out of his own pocket. You may find many other "extras" on an
airplane like this. We did.

Something that you can do right now is study the ADs for the airframe,
engines, and accessories. Learn which repetitive and or inspection AD's can
be eliminated by a one time repair or alteration. Finding repetitive AD's
that have been eliminated in such fashion is like finding money because you
won't have to have your A&P spend your money on those in the future. Again,
study the AD's so you know what to look for in the logs.

Jim