I just thought I would merge these two posts together, since they are the same issue.
Well Kate and Kirk, it sounds like you have a 30 year old Opel. I hope that you don't get too discouraged too soon. There will be lots of time for that later. Just kidding. Sort of.
Well, first things first. The leaking fuel pump. Is it leaking oil from around the base? If so, it might just need a new gasket, and a bit of gasket sealant. Make sure you GENTLY remove the phenolic (a kind of a plastic for old people) spacer (like a thick gasket) and re-seal on both sides. Be careful how tight you make the bolts, since they are threaded into the front timing chaincover, which is aluminum. If the threads get stripped (or are already, and hence the leak), you are about to learn how to install a Heli-Coil (it's an 8 mm bolt). Very easy, actually, but the first kit will probably cost about $20 (for the threaded tap and installation handle) and a few bucks for a correct sized drill bit. After that, the inserts are only a buck or so apiece.
If the oil leak is around the body of the pump, it might be worn out. When the plunger wears, it allows oil into the body and it leaks out of a little weap hole. They are $37 at OGTS, which is pretty cheap. You could also buy an electric pump, and get a blank made to seal where the old mechanical pump was. There are pros and cons to each.
As for the transmission rattle, it isn't likely related to the speedo cable. Rattles are hard to diagnose without actually hearing them, but it could be a bunch of things. Release bearing, a missing thrust spring (where the driveshaft fits into the tail of the tranny), a worn out pilot bearing, and all sorts of internal problems. Some of them can be fixed quite readily, others are easier to fix by replacing the transmission. Does it rattle all of the time, or just when you have it in gear and the clutch out, or only in neutral? Tell us more, and we might be able to help you.
As for choices besides the Getrag, well, you have none. Not really, anyway. I believe that the early Vega used the Opel transmission. But there are LOTS of pretty good Opel transmissions out there; all the 1.9 litre Opels, including the Manta, Ascona (1900) Kadett and GT used the same tranny. Just post here in the "Parts Wanted" section of the classified, and perhaps on the ClassicOpels list. Do you know any other Opel owners around you yet? If so, ask them.
As for the Getrag, it depends on how serious you are about keeping, and using, the GT as a daily driver. Most folks say the Getrag is the best money they spent on their Opel, after the Weber and maybe Pertronix Ignitor. Especially if you do any highway driving; that fifth gear overdrive is a pretty nice thing at 65 mph.
HTH