I was considering a 5 speed for the transmission
The Getrag (from the later Euro Opels, available from OGTS) is a terrific addition, allowing the engine to rev at a reasonable rpm on the highway. That said, there are better performance transmission choices, but none as easily obtained or fitted. Look under the Forum "Clutch, Transmission and Drive-train Upgrades" for lots of late night reading. A Limited Slip Differential (LSD) is a very good choice if you get an engine that puts out the HP, and some info is available on the same Forum.
I have read that a 2.2 is a good option, and would welcome input. Can a 2.2 be fitted w/ a turbo?
The 2.2 (or 2.4) is available from OGTS (or directly from Europe if you have the connections) and is a "bolt-in". Well, almost, depending on what you choose for a fuel system (the 2.2/2.4 has a different head, and requires either a grafted-in stock late model EFI system, or a never-seen-in-America carburetor manifold to fit that later head)
A turbo can always be done (on any of the CIH blocks, or even a swapped-in engine), but a search under the "Opel Engine Performance Modifications" Forum will show it is not for the faint of heart, as the GT is VERY space-constrained. Generally, a 2.2 or 2.4 (even a 1.9) can be made to perform to a quite high level of performance, without the complexity of the turbo. The darn car only weighs 2200 lbs dripping wet, so 150 HP can give just about anything else (short of a Mustang GT, Porsche Carrera or Corvette) a run for its money. Until the twisties, at which point a properly turned-out GT (or a fairly stock Manta/Ascona) will leave the domestics in their dust.
Is it reasonable to consider adding AC?
Air Con was a dealer-installed option on the 1.9 GT, which means that it can be installed on any 1.9/2.0/2.2/2.4 CIH engine. Unfortunately, there are virtually NO after-markets kits available that fit a GT (at least without significant fitment issues). Your best bet is to find a used AC package (the console and rear evaporator assembly; on eBay or here, or try OGTS or the other Opel vendors) and then custom-fit a newer, smaller compressor. I have heard it discussed lots, have yet to see one done, but it should be readily doable. More to read at "Group 9 - Accessories - Radio, Heater, A/C, Sound Systems". But a Turbo AND AC? I don't think so, Tim.
The photos don't show it, but the windshield has a BAD scratch from a worn out wiper blade, so I will have to replace it. Is it still available new? I am also looking for wiper arms. Scott said he never drove it in the rain, so he did not need them. hmmm...my rain storms don't always give a 24 hour notice.
Can the scratch be polished out? If the windshield is in otherwise good condition, that is far preferable to installing a new one. Yes, they are available new, from Locan glass, and some other good glass dealers. Used ones (and this week, even a new one or two) show up on eBay fairly often.
Wiper arms should be available from OGTS (frankly, I have never asked). They show up "used" frequently on eBay.
Oh, and use the "Search" feature lots, and impress us when you add a relevant question to an existing thread.
HTH and welcome, and good luck!