What is the first ring / amulet available and when do I get it?

For random loot, my first game I got a ring from one of the orcs outside Aleroth with a Repair bonus on it. I had one ring before then, but don't recall exactly when I got it; it was more likely random loot than purchased from George, though. I wore the ring with a repair bonus briefly, but found or bought another ring not too long after that (I kept any jewellery I found with Repair bonuses to use when batch repairing stuff to sell, as well as items with Identify, Trader's Tongue, Pickpocket, etc).

The first merchants that would have a good chance of carrying jewellery would be in Ars Magicana, far to the east of the farmlands after you finish in Aleroth. There are another couple merchants in Verdistis you can check (when you get there), though they would not have nearly the selection of the mage in Ars Magicana. Before then you will meet a travelling merchant on the path to the cursed abbey (north after you cross the bridge south of Aleroth) who has decent goods for relatively early in the game (don't recall if he has much jewellery), but you only get one encounter (I bought a nice upgrade for my sword from him).

There is an amulet at the bottom of the catacombs in Aleroth. I used it for the rest of the game, though if I hadn't put a gold strength charm in it (random drop by an opponent Zandalor kills) in order to equip a sword (dropped by someone near Stormfist Castle) I probably would have replaced it with the amulet from the Holy Item Quest late in the game. There is also an amulet in the cursed abbey (best to be in the late teens or early twenties for character level) and one in Stormfist Castle (may want to leave until you finish off the majority of Rivertown, including the sewers, and Verdistis... some people also recommend exploring part of the Dark Forest first).

One of the two soldiers you can heal in Aleroth will give you a ring when you reach Verdistis. Another ring can be found in a house north of the cursed abbey. I used these two rings until the end of the game.

As always, stats on items consistently in the game are still random. I saved before looting, or checking my inventory for the ring in Verdistis, then reloaded a few times to get good stats, saving if I found something decent but wanted to reload again (for items placed directly in your inventory, you need to move them in your inventory to set their stats, not just identify them).

Thank you. I have another question though. Do summoned NPCs despawn and do NPCs health points reset when you leave the floor / room they are on / in?

Summoned enemy creatures stick around when you leave the level/area, though summoners stop summoning. NPCs may heal if left for awhile, especially if they know a healing spell or there is a healer in the group (like the orc drummers outside Aleroth). It isn't a simple hit point reset when you take stairs or teleport away, though. The game doesn't draw NPCs on screen that are too far away to show up on the mini-map, so presumably there is some kind of time based healing calculated when you return to an area where there were injured opponents.

Your own summoned creatures will also hang around when you leave the area (and continue fighting for awhile if there are opponents left), unless you cast Aura of Command on them to get them to follow you. Resurrected creatures will turn hostile if you leave the immediate area (in the download version of the game they will die if you move to a different map region), and can be re-killed when you return (for more experience and loot).
IIRC you may be able to select the healing spell as your secondary skill and right click on your allies to heal them (I think someone said you can cast Aura of Guarding and Spell Shield on them, as well).

I think at least your own summons die when you sleep somewhere.

Most of your own summoned or resurrected creatures, and scorpion traps, will die when you sleep. At least in the disk version certain creatures would survive (the summoner from under the warrior's guild, dragon riders... maybe Shrimpo IIRC). Someone in the old forum had a topic called 'my boss collection' where they listed some similar creatures. They also had a screenshot with the warrior's guild summoner following them (which summons skeleton summoners); after coming in range of an enemy, there were a lot of summons quickly, pretty much filling their mini-map with green crosses.