Folding Bikes - 20" Wheels

I need to find some 20" alloy wheels to replace the original wheels on my recently acquired Raleigh Twenty. I will be setting it up as a fixed/SS using a flip flop hub. I have been looking at various BMX wheels listed on Ebay. The narrower rims that would be most suitable for my intentions all seem to have a weight limit. 90 pounds or 100 pounds. Generally 36 spoke. Why would these rims be limited so? If I bought similiar rims, spokes and hubs and built my own wheels would they also have a weight limitation. And is the limitation cummulative? Meaning does two 90 pound wheels equal 180 pounds? What are other possibilities for a lightweight, inexpensive 20" wheelset?

You should be looking at recumbent wheels rather than BMX wheels. Here is a good source:
http://www.gaerlan.com/bikeparts/bike.html

"Inexpensive" is a relative thing. This is nice, but out of my budget. Anyway it's too small for a Twenty:-)
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=58098&item=7111599939&rd=1

The narrower rims that would be most suitable for my intentions all seem to have a weight limit. 90 pounds or 100 pounds. Generally 36 spoke

Are you sure that that limit is not pounds per square inch and merely a guide to maximum recommended tyre (tire ;) pressures?

Thanks for the tips.

I like the thoughts of the Pantour suspension hub listed at gaerlan.com. That might negate the need to go to a front suspension fork ala Sheldon. Though they are expensive, it still might be cheaper than a new front wheel, fork, and stem.