I have a Suntour BL (blue line) GT RD on my Univega Turismo. That would be a good choice. It should clean up nice.
I have a Suntour ARX on my Lotus. I thought it was the GT version (long cage). I'll take a look when I get home. The ARX are pretty decent components as well.
This is a day 1 pic of the BL derailleur (before cleanup and dumping the pie plate).
There is a Suntour ARX on ebay right now, looks like a long cage. Lousy pictures, may be in mediocre condition. But there are several other Suntour long cage choices on ebay right now.