The Schwalbe Racer is 1.5" and it don't last as long as the Marathon Plus or Supreme. Is it because of the treads? No worries, the Supreme and the Plus is pretty good too and the rolling resistance , I think the Supreme is much better with less treads. If you read through a website, actually foldie or roadies tires are too narrow to actually make a difference in tread patterns. Most of the time, just a narrow part of the whole width is actually in contact with the tarmac, so the treads makes no difference, unless you are talking about wider tires like the Big Apple or the Maxxis DTH. More important is the rubber compounds that makes the tires. Even the bald Kojak sticks to the tarmac but the compound is sticky when hot.

I think the original rims should be able to handle all three models of the Marathons though. But the 1.5" tires on the 1.5 wide rims will lower the bike just a bit....just a bit.

If you can find 7 speed 11-32t freewheel cogset, you are in luck! You may/may not have to lengthen your chain. You RD could most probly still work. But u r most likely to find 14-34t
Won't cost you more than 150
Changing to double chainring will cost you a bit more. you need new Crankset, new Front Derailleur , new front derailleur hanger and left shifter,  will get you in the 500 ballpark either if you use used parts and  don't go for the "finest" components.
Another alternative is to change to 8-speed 11-32t(or 34t). Freewheel cogset, 8 speed right shifter , under 200 kowtim.