I have a Twin Famicom (plus other Fami units like the Fami Titler) and for reliability, you should buy a Twin Famicom that has had its belt fixed already. Let's face it, belt changing & calibration is a minefield of problems for newcomers to the FDS, and since most Twin Famis come out of Japan anyway, a good seller/shop should be fixing the belt themselves before selling.
Definitely, Ai Senshi Nicol is one of the FDS' best games, along with Zelda, Doki Doki Panic, Arumana no Kiseki, and Gyruss.