In theory sugar should work just as well as salt but I think you would need to be more concentrated and find some flavorings that go with the sweet lemons, vanilla would be interesting.
Bearing in mind that you normally only use the rind of the salted lemons (they use the pulp in Berber cooking I think) what would you use the sweet ones for? Might be nice if chopped finely and added to something like a lemon meringue pie. They would add an extra burst of flavour and some additional texture.
With regard to picking your lemons, try to avoid the really thick skinned waxed ones. Best to go for smaller, thinner skinned, unwaxed and if possible organic. Do try with some additional spices such as cumin seeds, bay, cinnamon stick, and pepper corns, it makes so much difference. In the past I have cut down on the cost of additional lemons for juice by mixing up my salt and spices, quartering the lemons and packing them very tight lemon/salt/lemon/salt until the jar is full. But then the lemons are so packed in you only need the juice of one extra lemon to fill to the top. It also means you can use normal jam jars rather than expensive large ones.
I will look up the recipe with the spices and post it for you to try