You can not combine weapons. If there is an enchantment on a piece of equipment you can have an enchanter remove it to recover the formula (but not the ingredients), and then enchant other equipment with it.
If equipment has a charm slot you can permanently add any charms you have found/bought.
You can save before special chests or turning in certain quests, and then reload until you get something you want. Whenever you level you can do this with merchants, as well. Except at the start of the game or certain situations this can be a waste of time, though.
For merchants or lockpickable chests the contents are set when you get close and run the crosshairs over the them; with locked chests that need a key (or password maybe?) I think the contents are only generated when the chest is opened.
Usually you just need to explore everywhere, do quests, etc, and you should find enough good equipment with random loot or rewards.
