Nice pics! It looks like you have 9/10ths of what could be a good-looking, reliable and decent performing road car that's within a reasonable budget.
Knowing what I know now, I'd find the cheapest way to get the motor running and the car on the road. If your motor's bottom end is seized, or if water in the cylinders caused
hydro-lock, you might be better off dropping the entire engine/transaxle assembly and replacing the whole thing with one from a wrecked car in a junkyard. Just make sure that the yard guarantees the parts to be in working condition before you buy!
As far as the leak goes, I hope that you have a sunroof, bad side window seals or something other than the bodywork that's causing the leak. Something that a new set of seals will fix. A car of that age shouldn't have holes in the sheet metal. If it is rusted away that much, you could be better off selling the whole car for scrap, and looking for a better project car. Don't become too attached to the thing.
Don't know how to weld? Get some scrap steel and practice. Oxy-actelyne welding isn't very popular these days, now that electric arc welding (including
MIG and TIG) have become affordable. But if gas welders are all you have, you might as well use them. A good first project is to build your own rear strut brace. Stuff like that can help handling, and the raw materials are cheap enough that you can toss them and start over if you make a major mistake.
Another thing you can do to get free horsepower is to do a little grinding and polishing of the intake manifold to improve air flow. You can also dress up the engine compartment with polished metal intake tubing and a K&N air filter to replace that ugly air box. You can buy intake kits with various pre-made elbow bends. All you need to do is mock it up, take measurements, cut the tubing as needed and use silicone rubber sleeves to join the plumbing. You can build your own sheet metal heat shield and brackets to separate the air inlet from the rest of the engine compartment (an ideal thing for oxy-actelyne cutting and welding). Since this costs next to nothing and makes use of the tools you have, it's good practice that doesn't keep the car out of commission for days or weeks.