I've played Sacred only in single-player mode, and finished the campaign on Niob difficulty. So, yes, it is viable. No, runes regularly drop from killed monsters.

If you want to enjoy it Solo, there is nothing really stopping you. You won't max the skills (but wll find enough skills, don't worry too much), and won't have the best equipement (but you will do good enough with what you'll get). In the end, Sacred is correctly balanced for a Solo game. If you feel like using the save-load thing for quest reward, it will help you of course but it's not needed.

Actually in single my main character is now level 35 and I ge ta surprisingly large amount of Set Items (green) from random quests. I would be interested to know if this is a change introduced in the "Community Patch" or just the way the game always behaved?

I have found so far on my lvl 35 melee Dryad, without using any cheats or load/save thing : 5 pieces of Detheya's, 4 pieces of Daerwain's, 5 pieces of Leaves and Twigs (a C.M. set I think?), and another 3-4 usable sets.

I also received 3 sets pieces for other classes so far. It seems the Set Item rewards are weighted in favor of the class you play.

Quite a few uniques as well.

The drop rate of unique and sets is far higher than in Diablo II.

OP was talking about Sacred 1.

AFAIK drop rates for set items weren't changed in the S2 CM Patch. If anything completing a set will be more difficult because the pool of possible drops is expanded.

And all items are weighted in favor of the class you play, armor, weapons, and runes.