Questions are 1) Is it indexed or not and 2) is it a cassetted hub or not?
If a new chain skips on the old gears, you're going to have to replace them as well. If it's friction shifting and a freewheel you'll have minimal trouble, just replace the freewheel, they're readily available in 14-28 gearing.
If you have indexed shifting and a suntour cassette hub, you're in for more trouble. You will either have to find a suntour cassette or buy a new wheel. Since it's not necessarily easy to find Suntour cassettes, especially on the road, It would probably be best to buy a new wheel (Shimano does have the best options for commuters and tourists). However a new, non-suntour cogset in the rear is not going to work well with a suntour shifter, and possibly not the derailleur as well. In other words, you may be looking at a new wheel, cassette/freewheel, rear shifter, and rear derailleur in order to make everything work right.
My advice? Don't fix what ain't broke. If the chain measures OK and there's no significant wear on the cassette/freewheel, just use it and maintain it. A new chain would be fine for an emergency, but it may not be a permenant solution (sheldon brown has an excellent article on chain wear and replacement on his site).
When looking to actually upgrade, I would suggest a wheel with a cassette hub and replace the shifter with a friction shifter. It's not a convenient as indexing, but it removes compatibility issues and are much less suseptible to dirty cables, drivetrain wear, and accidents; all pluses for commuters/tourists in my book.