I just looked up the Simplex SX610 RD that's on the bike and there's two versions. The GT has a total capacity of 38T (
link1 link2) and the T version doesn't say. If it's the GT I think this will work. The bottom bracket spindle may need to be changed and I don't know which length I need, but velo orange sells a french threaded sealed cartridge bb in variable lengths for $50, if it is indeed french threaded. If all I need to change is that and the crankset I wouldn't mind doing this. Do you think it's worth the risk to just buy the crankset now? The ebay seller just offered $5 off lol.
As for the wheels, they have 700c tires on them so I assume they are 700c wheels... Shifters are friction, and I don't mind keeping them as is. I'm leaning toward just buying the triple crankset and playing around with it.
Pictures - ignore the cheap saddle, pedals and my mess
replaced the jockey wheel on here a while back as it had a broken tooth that's why it's a different color.