Welcome, George.
I'm at about the 80-85% point on my first time coupe build, and I believe that the biggest budget buster is lack of planning, and even more, changing direction or making major modifications in your approach after you've started. Look at the more experienced builders here, the guys that have been doing custom rides for a long time, and they have detailed plans up front - drawings of the final "look", stance/ride height, and all major (and many minor) components nailed down before they ever start; or they may be building a basic unit from experience, changing details to make them unique. Bottom line, they know up front exactly what they are going to have when they are done. My biggest hit was the power train. I thought I would get a takeout engine/tranny from an old Mustang, and ended up with a custom 347 stroker topped with a new Edelbrock EFI, all coupled to a brand new Tremec 5 spd. It's all great, but a major hit to my bottom line. Another consideration is tools. I had a lot, but I also added or upgraded a lot as I went along. Many things can be found cheap, like my portable hoist from HF, on sale $199 less 20% via coupon gave me 2T lifting capacity for $160 plus tax. Still it all adds up.

All said, I would do it again in a heartbeat! I love the fabrication and problem solving, and the satisfaction of seeing it all come together cannot be beat. I would advise you to plan, plan, plan, and when you think you have it all nailed down review it from the start, see what you missed, and then detail as much as you can into a committed plan that you stick to. I'm into interior stuff now, and I just picked up a fantastic book, Custom Auto Interiors by Don Taylor:
http://www.amazon.com/Custom-Auto-In...1217473&sr=8-1
I wish I had bought this book before I started, as they point out so many things that need to be considered in the planning phase so that when you're ready to finish you've accounted for the details, or so that you don't find your head banging the roof and have to start over with seat frames. You'll see many of these same tips in the build threads here on CHR.

Sorry for the long post I can't help you with a glass sedan recommendation, but I would consider a roller chassis for a first build. Best of luck, and include a camera in your budget - everyone here is going to want to see pictures!!