There is a reason the Custom Audiosound products are priced so low, they are built cheaply. Just about every vintage car I have bought has had a Custom Audiosound receiver in it when I bought the car. Not one of those C-A systems were working properly, unless you needed a static generator. I removed every one of them and threw them in the trash. RetroSound seems to have a better reputation these days. Go to the Crutchfield site and look at the various options available. You did not say which features you want, the prices vary considerably. If your criteria is “absolute lowest cost”, you probably won’t be able to beat that C-A bundle.

This is a fairly loaded questions and requires a bit more information.  What are the speakers you have now?  critical things to know are resistance (in Ohms, & anything over 4 is junk), RMS, and Max power handling.  There's no point over amping crappy speakers; they'll just blow.  If the speakers are decent and you under amp them you'll have to turn the gain way up on the amp to get them to make sound and you'll kill the amp. 

If you don't want to thump there's no need for a subwoofer.  I am a bit of an audio snob, but even I don't run a sub in my '67.  A decent set of 6x9s will provide enough thump for most guys.  The key word is decent; they need to have max power handling of at least 150watts and frequency response down to at least 50Hz. 

From what I've seen the audio stuff designed to fit into classic cars is all junk from an audio system standpoint.  I elected to just modify my radio bezel so I could run a standard DIN chassis Alpine head unit in my '67.  I used a JBL component set in the front (mids in the kick panels, tweets on mounts on the A-pillars I built), 6x9s in a repro trap door I hacked up & sealed, with a Rockford Fosgate amp to run the whole thing.  Such a head unit isn't going to fit in a '66 without hacking up the actual dash, and I wouldn't recommend that.  BUT, there are kits that allow you to hide a standard head unit and control it from something that does fit in the stock location, and that's where I think I'd go. 

After cruising with my system for a year I decided that I need a sub.  I think at this point I'm spoiled because all my other cars, except my truck, have a sub, and I've gotten used to that low end punch.  I went with a Rockford Fosgate 10" single sub in a Rockford Fosgate built sealed box.  Buying through Crutchfield I was able to save some cash getting a "bass bubndle" with a Rockford Fosgate amp.  I'm in the process of the install now.  The box isn't too big and doesn't take much real estate from my already tiny fastback trunk (I think its listed at 12x16x16, but the back in angled, so it takes up even less space than that would suggest).  I had to build a plate to mount the second amp, which I'm running out of the first amp (4 channel with a low pass crossover specifically for this purpose).  The 2nd amp bridged will produce 300 watts RMS and 500 peak.  I'll fiddle with the gain until I get it balanced, but I'm running my current amp at 5 rear, 4 front (out of 11).  I also have adjustable gain via a switch that I'm thinking of where to mount.  That will allow me to adjust the gain (more bass) from the front seat for different kinds of music if I want a little more thump. 

All in all, with audio equipment you basically get what you pay for, up to a point that the average person can't tell the difference.  I probably have $1,500 now tied up in my whole system including wiring, kick panels, etc.  Its not going to win any audio competitions, but it will sound awesome blasting down the road with the windows open, and I won't feel like I'm driving something from the '80s with a crap factory system. 

My experience with the "custom fit" stuff for the classics is that its junk.  If you care about audio buy real components and figure out how to make it fit.  Frankly, its not any different than what we used to do back in the day when there were oddball factory speakers (GM liked 4x10s in their trucks, others liked 5x7s, etc.) and an audio system was an afterthought for a manufacturer.  If you take some time and put some thought into it you can come up with solutions that don't involve cutting holes in the car.  I could swap my whole car back to stock and leave no trace except a few screw holes in the trunk easily concealed by a new trunk mat.