Regarding the rudder:
a) if you do a good job, no one will know it was homemade;
b) a first-time buyer is looking for a rudder, not a professionally-built rudder;
c) if money is a problem, make one and start saving up for the "real" thing; and
d) if you really don't want to make one, check eBay and Craigslist. You may need to move the pintles but I'm sure you can find something that will, at the very least, tide you over.
Regarding the rest of your experience:
welcome to dinghy sailing! That's what happens with small boats. You learned a lot of very good lessons. If you decide to go with a mast float, I've heard of everything from tying some empty milk jugs to the mast to going with manufactured floats like
or this one
From a bigger perspective, I would not let this experience stop you from singlehanding. You are correct, you need to adjust your weight as the sail moves. Your position fore and aft, as well as laterally, make a huge difference in how a dinghy sails. Your boat probably weighs somewhere in the 200-400lb range. So, if you're an "average" guy, when you are aboard you increase the boat's weight by somewhere between 50% and 100%. Envision your boat as a see-saw (or teeter-totter) where the pivot point is around the centerline of the boat. On one side of the see-saw is your rear end, and on the other side is the sail which is full of wind. The sail balances out your weight (or vice versa). As the sail approaches the centerline of the boat, all of the "weight" (or, more correctly, the forces) shift from being somewhat balanced to being highly unbalanced as all of the forces are on one side of the see-saw. If you stay aware of the sail and pretend your body is the mirror image of the sail, and you'll help keep it from going over. It's not a 100% guarantee, but that's the easier way to do it. When your friend was aboard, I assume he sat on the opposide side of the boat as you. The see-saw in that case was never completely unbalanced, that's why you had a more "stable" ride.
More importantly, though, why gybe? In my admittedly limited experience, that typically involves much more signifcant forces and much faster movement than a tack. Stick to (mostly) tacking as you single-hand, and life is simpler.
None of this is a reason to not single-hand.