The 28 tooth low is the minimum I would suggest you consider. There's no way to tell if that's sufficient, as "average shape" means little when it comes to cycling. The problem is that we don't know if your derailleur will handle the 28 tooth cog. It's not "clearance" but rather "capacity" that is the issue. You need to find out what model of derailleur you have, or at least take a picture (from the side, at the level of the derailleur). If the bike is friction shift then changing both derailleur and freewheel is still not that expensive, and you can go larger than 28 teeth. But even a 42/34 may not be low enough for "any hill" you may encounter.

I would not recommend any chainwheel upgrades without knowing more about the bike, as you may end up putting in much more money than is advisable.