1) Go for 20" wheels - more practical if you are going to be doing longer rides.
2) Agree, Downtube or Dahon are good choices for a first folder & on a budget.
3) Caution, folders are not designed to be Mtn Bikes - unless you put down the bucks for a Bike Friday Pocket Llama.
4) In Portland/Eugene area you have the opportunity to see and test-out lots of folders - Coventry Cycles has Dahon & Bike Friday, Clever Cycles has Brompton, Joe Bike has Abio, and if you go down to Eugene (120 miles to the South) you can visit the Bike Friday factory and the Center for Appropriate Transportation where one of the co-designers of the Swift makes the steel version - http://hpm.catoregon.org/?page_id=214
5) Would suggest you look at an array of choices, so you can better evaluate the "entrance level" models.

Lou