It may be a Miyata-made frame, but I would not enlarge the rear triangle. It should be just fine for you just as it is. Unless you are pounding lots of hills, should you consider doing a upgrade such as that.

This bike will ride differently than the Antelope. You'll have to get used to the new shifter position, as well as the drop bars.

I'd cut the cables, remove and clean the parts you can (brake calipers & shifters), remove the bar wrap and then go to a place where they will help you restring it and get some new bar wrap on it. (If the brake levers on it are scuffed up you may want to find some better looking replacements before you do the bar wrap).

The mechanics forum can be helpful, and there are instructions and tutorials on how to do stuff on the Park Tools website, and other sites too. An older "Complete Guide to Bicycle Maintenance and Repair" by Bicycling Magazine is a good book with lots of photos and thorough instructions.

There may be some bike co-ops in your area that will help you do the work for minimal cost and parts are not expensive. Also, I know of one chain of shops here in northern California that sets aside a few hours a week for customers to come in and the mechanics will help you fix your bike. Parts aren't free but basic things like cables and brake pads, etc. are not expensive. You learn while you are there and you have the comforting knowledge that a real mechanic helped you through it.

Or one of the C&V members here may be local to you and would be willing to help you out.