If you want to stay Raleigh, I don't think there's much difference between Sprite, Record, and Grand Prix frames; Grand Prix has fancier lugs and wrapped seat stays (purely decorative), Sprite has fenders and (sometimes) chain guard; all were made in 25" but who knows which you'll find first. Certainly a higher end model would be nice if that turns up first.
I built a 5 speed around a Lambert frame, with "almost matching" Free Spirit fenders, which came out quite nicely. Really, any frame with horizontal dropouts should be fine.
Are you going to keep the 2 cog thing? I'm going to be trying that myself pretty soon.
I thought the Record only came in a 23--I had one in 1970, but maybe I'm remembering wrong.
Definitely going with the two cogs again--my age and the terrain demand it. Suprisingly easy to set up, and works flawlessly--if you choose the cogs right you get a nice half-step between the big S-A jumps.