I just went through the exact same thing you're doing while building my wagon. I did a lot of homework on all the parts I bought and was on a budget so I always bought from whoever had the lowest price for what I needed, unless it was something special. Here's some suggestions:
Valve springs: Comp Camps 980-12 from Summit Racing
Valve seals: I bought a set from Poston's but they sucked, didn't fit tight enough on the valve guides, so I kept the original ones on. Afterwards, a couple guys said to use OEM seals.
Valve keepers: Poston's
Timing chain: I bought a new all-steel OEM-style (non double-roller) chain and gears from a member of this board. I forget the brand but think it was TRW, nothing fancy.
Timing chain tensioner: Poston's.
Gaskets: From Poston's and John's Performance.
Motor mounts: new stock replacement units.
Oil pickup: didn't touch it, just cleaned the screen thoroughly while the pan was off.
Oil pump: oil psi was good so I didn't touch it, but added the "biggie" filter adapter from RJC Racing.
Front main seal: neoprene unit from Poston's.
While you're at it, you should also replaced the turbo oil feed and drain lines. I used the braided ones from John's Performance. This is also a good time to add oil psi and water temp gauges if you don't already have 'em, as the sender installation will be lots easier with the motor out. I used VDO gauges from
www.racerpartswholesale.com (great prices). Also a good time to replace the water pump if yours is old (mine wasn't so I left it), along with the belt and hoses.
I also installed a mini-starter (purchased through
www.turbobuicks.com for $125) and new battery cables and a ground stretcher kit from Casper's. Also new spark plug wires, coil pack and module, and numerous other stuff purchased new or "lightly used" from guys on this board.
You probably noticed I purchased a lot of stuff from John's Performance (
www.johnsperformance.com) out of Washington state. He has a great website, quick competent service, and pretty much the best prices on everything he sells. Highly recommended.