A sorcerer will have almost as many hitpoints as a thief/illusionist multiclass. 60 instead 70 for the first 10 levels and both get 1 HP per level up. As single class character a sorcerer will level faster than your thief/illusionist.

Items which work for mages work for sorcerers too.

One big difference in HP is that I now have UAI, which means I can use the bard item that gave extra HP for example, which the sorcerer can't use. But other than that it's nice to hear he'll have pretty sturdy HP.

Cool about the mage items too. Wasn't sure that would apply since the spell mechanism is so different. How does it work when I get to HLAs? Do these have to be picked among the normal level 9 spells? Can I then instantly cast as many e.g. Dragon's Breath as I can, for example 6 at level 20?
(According to this table: http://www.pocketplane.net/volothamp/spells.htm )

If you pick a level 10 spell as HLA you can cast it as if it was a level 9 spells you picked, it's simply added to the known level 9 spells in your spellbook and you can cast 6 dragon's Breath in a row if you wish. Therefore sorcerers benefit even more from the level 10 spells than mages.

Nope, as a maxed-out sorcerer you'll end up with 4 normal 9th level spells and all of the HLA spells. The HLA spells don't count towards the spell knowledge limit. You'll only have your normal number of casts per level, but you'll end up with twice as many 9th level spells as any other level.

Thanks, that sounds quite excellent. Think I'll give sorcerer a go next to see what it's like.

I'll put in a word for a cleric/fighter or cleric/ranger as a good class for a new person. True, your weapons are limited and you'll level more slowly, but you get to wear full plate just like any other fighter, and you'll get to cast healing and buffing spells. You get the spells automatically and don't have to purchase them (though you still have to pick which you can cast each day), and if you play a cleric/ranger, you'll have access to Druid spells as well as cleric spells.