I'm kind of partial to this style wheel on the Avalon:
http://www.americastire.com/dtcs/fin...5248&rc=TXWINT
It has the look of BBS RX without the BBS price, which is probably overkill for our aging daily-driver Toyotas. You can also get it in dark gray or black, if you really like that look. Drag wheels (and some others in this price range) are "OK" in quality but not as good as the better brands. You can spend up to $400-$500/wheel but how good do they really have to be? Actually, you can probably find pretty good prices on BBS and other aftermarket wheels on eBay if you search. You can also shop CraigsList; some Lexus wheels are supposed to fit.
Anyway, Americas Tire (AKA Discount Tire) and Tire Rack are good sources for a variety of wheels and they both give you the ability of seeing what the wheel you select looks like on your car.
IMO 18" is too big a wheel for the 1st gen Avalon and would give it the "wagon wheel" look when paired with low-profile tires, especially if you don't plan on lowering your car. Stock is 15"; I'd tend to go with 16" or maybe 17"; beyond that I think you'd be messing up the ride and taxing the suspension (already a weak point on these cars) beyond its original design parameters.