It may be the way to go if you're looking for a custom project on a budget. The Twenty is a good frame and has a very renowned ride to it, but it is not the lightest nor the most modern. You can find more information about projects using this bike online if you are interested, as there are many fan sites. Sheldon Brown's site has detailed information on the subject:

http://sheldonbrown.org/raleigh-twenty.html

Contrastingly, the Xootr Swift frame for instance is much more expensive to buy than an old Raleigh, but the frame will be new, lighter, and it will be user friendly regarding using modern, mainstream components as opposed to the Twenty, which requires some hunting and sourcing and effort in order to customize beyond basics.

I think it's worth noting that there are lots of different types of folders, and some serve a purpose better than others. A Bike Friday for instance, is a very high performance folding bicycle - but for pure size and convenience of folding, it falls far behind most other choices. A Raleigh Twenty is a great choice if you enjoy vintage bikes and don't mind the folding size, but a Twenty is not a great bicycle from a pure folding perspective either.

What are you planning on using your folder for? It might help narrow down which models would be better.