For Metric sockets I'd go grab the complete Toptul noskip smorgasboard off eBay.
http://www.ebay.com/itm/TOPTUL-GAAQ...191069?hash=item1c76801a1d:g:9RYAAOSwHoFXsGWw
I have SK and Wright and GearWrench and I almost always grab the Toptuls.
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Hell of a deal I feel for 70$.
For SAE sockets I'd go for GearWrench or Williams Taiwan. They used to show up all the time in the Amazon Warehouse Deals thread on crazy deals but I'm not sure if that's true anymore.
Impact sockets = Sunex
They are synonymous.
For combination wrenches I love my WrightGrips, but you really don't need to spend that much unless you just love tools. I've honestly been surprisingly happy with the DeWalt wrenches I have picked up when I'm out somewhere trying to help someone and they're missing a size. If you can snag them on sale at your local Sears or Farm and Fleet they'd probably be a good option.
GearWrench ratcheting combination wrenches are on sale all the time at Advanced Auto Parts. They're a good deal but watch for skips, most of their sets are very skippy.
Wera, Felo and Wiha are all top end screwdrivers, and you can get them all with striking caps which can come in handy working on cars. Surprisingly though, I picked up some of the Kobalt striking cap screwdrivers at Lowes when they were clearanced earlier this year and I end up grabbing them a lot. I really like the big square handles. They were not expensive at all (3-5$ a piece).
Beyond that telling us what gaps you're looking to fill would probably help. Also do you have the non hand tool garage essentials? Compressor, floor jack, jack stands, impacts, that kind of stuff?
Also do you live near a Harbor Freight? As long as you stick to the "gems" people around here can recommend you can fill in a lot of the holes with quality shizzle wizzle. For example, I splurged on a 1/2" 30" SK XXL breaker bar. It's beautiful and awesome, but I don't feel like it's stronger than my 1/2" HF Professional breaker bar. All of my breaker bars have been replaced now though by the ultra beast 1/2" Wright flex-head ratchet I recently got. It's preposterously beefy.
Edit: I just saw your post that was made while I was making mine about shearing the breaker bar. I think HF quality has improved quite a bit on some of the hand tools over the years. Their Pittsburgh Pro ratchets are very nice now.