My bike is a 1995 Trek 5200. It is mostly stock components (Shimano 600 Ultegra "tri-color" 8-speed group throughout), as specified here: http://www.bikepedia.com/quickbike/b...5200&type=bike

I have upgraded recently to new Shimano 6700 wheels (which are compatible with Shimano 8/9/10 speed cassettes), and I'm currently running a Shimano 8-speed 13x26t rear cassette (which was not stock).

I would like to replace some component(s) in order to get lower gear ratios and need to understand my options.
1) How do I determine the minimum and maximum gear size that the rear derailleur can accommodate? I've been told that it is a "short cage" derailleur.
2) I'm finding rear cassettes with a wider range than my current 13x26 very hard to come by. Many of those I find are intended for mountain bike use. Would XT or XTR 8-speed cassettes work on road bike hubs? Would I need to replace the rear derailleur to accommodate one? Would my current STI shifter be compatible?
3) On the crankset I've got 53/39t. Can I simply replace the smaller gear? Are any gears like this still available? Would this be too big a jump in size? Would this be compatible with my existing front derailleur and STI shifter?
4) Are there any compact cranksets available for 8-speed systems? Would they be compatible with my existing front derailleur and STI shifter?
5) I don't want to replace the crankset with a triple, as this would necessitate changing out my front derailleur and STI shifter too. By the time I pay for all that, I would be better off investing that cost into replacing my entire drivetrain with a modern 10-speed group.

Any opinions regarding my best options here?