At level 7 with a good team and some light feeding my Oak can have 5500 hp for 5300 gold.
Here are the items he uses (courtesy of demigoddb.com):
+2900 Health
- Banded Armor (+400)
- Blood of the Fallen (+800)
- Hauberk of Life (+600)
- Nimoth Chest Armor (+500)
- Unbreakable Boots (+600)
He hits 6000 hp at level 11, with no further expenditure. That's endgame in most cases, but if we take it to level 18 he comes within 20 points of 7000 hp.
He gets 25 hpps regen from those items too.
+25 Health Per Second
- Banded Armor (+5)
- Blood of the Fallen (+5)
- Hauberk of Life (+10)
- Unbreakable Boots (+5)
We haven't considered armor at all - that item build at level 7 has 8250 armor-modified hp. At level 18 he has 11,300 AM HP.
You can do this with every DG, they all become really hard to kill, and they can all do it for little more than 5k gold.
A total item rehaul would be nice but it's dubious - the simplest way to fix HP stacking is to increase the price of HP items on the low end by about a third.
Quick examples of the new prices:
Unbreakable Boots - 2500g
Banded Armor - 750g
Nimoth Chest Armor - 2500g
And I'd make Blood of the Fallen a flat 20% HP increase so that it stays powerful but scales better. To give an idea of what I mean - TB at level one has 1410hp. If he takes Blood it gives him a 56% hp increase right out the gate. That's too much too early.
That means you can still buy your hp items but you actually have to pay for them now, and it gives DPS stacking relative value while keeping nukers in the game