Have the A&P mechanic do an annual inspection but skip the prebuy. He is not accountable for anything he tells you in a prebuy report. He is accountable for one year for an annual inspection which is much more thorough than a prebuy.

I have the Collier / Fletchair STC on my AA-1B.
As far as the Gross Weight increase on an AA-1A, this involves upgrading the main gear to the same as the AA-1B / 1C. Same gear legs but with the addition of the spring plates and probably replacing the brackets that attach the gear to the spar. Fletchair should have all the parts to do this. This gives you a GW of 1560 pounds, which most of the increase is used up by the heavier engine, prop, oil cooler, and main gear leg upgrade (those spring plates are heavy).
Also make sure the nose strut was changed. The engine STC calls for the -507 strut from a Cheetah / Tiger, but could be done with a -506 strut (late AA-1B/C and Traveler strut) if you can show adequate prop clearance.
I can’t help with finding a Grumman mechanic in your area, but Bill Shephard I believe is based at Deer Valley and he has been rebuilding an AA-1B. Maybe he has some contacts that could help. Look for his reports on his project in the Restorations forum.

So this plane has an empty weight in the 1150 range? That is a bit high.
Yes, look at the log book entries on when the STC was done on how much ballast was installed.
As a comparison, my AA-1B has an empty weight of 1113. This is with the 0320-A2B, no wheel pants, stock starter and alternator, battery on the firewall and about 7 1/2 lbs of ballast under the vertical fin. The AA-1B under this STC has a GW of 1600.
If you have more then say 10 or 12 lbs of ballast then consider moving the battery. Down side is it makes the battery hard to get at and service.
On removing the interior, I don’t think you gain much by doing that unless a previous owner did something like cover the old plastic with fiberglass or some material to cover the broken plastic.
The factory wheel paints are something like 17 pounds with all the mounting hardware I think. Then if all the Tiger fairings were added (gear leg, brake fairing, etc.) that will add up.
On my AA-1 Yankee project I plan on moving the battery aft, partly due to the different airfoil on the AA-1 where the center of lift is further aft requiring the CG be back further. I hope to use either the TSO’ed Odyssey or ideally the EarthX battery back there if my IA thinks he can get approval to do that.
See the pictures of the AV30’s I just put in the Yankee panel.
I am hoping to see the Yankee weigh in around 1100 pounds or less, using David Fletcher’s Vans RV style wheel paints and Tiger clean-up, no vacuum system, and little or no ballast in the tail using the 0320-E2G.
If you buy this plane, be sure to post pictures!

I have not weighed the components, but the consensus is a savings about 10 lbs.
Great looking bird. It has the DMA wheel pants, not sure how they compare in weight to the factory pants but would guess they are lighter.
If those are the rubber main gear leg fairings, those are a bit heavy.
Piper spinner which was an option on the engine STC.
CY & P have no meaning to me.
I owned for a time AA-1A N9300L which was painted in the same red with white trim as that, so guessing that is original paint with decals added. Looks good!

Part of the problem with the AA-1A is being the first Grumman certified with the “Cuffed” wing, I don’t think American fully tested the flight envelope. The rear side of the envelope moved up to 80″ from the Yankee 81″ due to the airfoil change, but the forward end of the envelope stayed the same as the AA-1. This gave the AA-1A a very small CG envelope. When the AA-1B was developed the forward CG was expanded, with no change in the aerodynamics of the airframe. To my knowledge no one has ever tried to addressed the narrow CG envelope in the AA-1A through an STC. Do I think it is safe to operate and AA-1A with within the CG range of the AA-1B at the same weight? I would think so, but it would not be legal. See the attached graph of the AA-1 series CG ranges.
At the STC GW of 1560 in the AA-1A, the CG envelope is only about 1/2″ wide. In my AA-1B which went to 1600, the CG is about 1 1/4″ wide.
In the AA-1A and -1B, you do not want the CG to get behind 80″, the aircraft gets very pitch sensitive, and stall characteristics may not be to your liking.
In my AA-1B at the forward CG limit I still have plenty of elevator authority on landing and can hold the nose off.
The last item on the AA-1A the limits the max GW is the wing spar material was changed when the AA-1B came out. For this reason, you can us an AA-1B wing legally on an AA-1A as a replacement, but you cannot use an AA-1A wing on an AA-1B.