$400 is a good deal if it is a G20. I picked up a G21 from a friend for $375 and that's an absolute steal. You def want to see if it is a 20 and hold it first. The 20/21 grip is rather large. Medium to small handed men generally find it too large and most women do not like it at all.
The 10mm ammo will cost much more than comprable 9mm ammo. The 10mm is the same projectile that is used in the .40 SW round, just a larger case.
Larger case = more powder = bigger boom = higher velocities.
If you reload and already have .40 dies, you're set. If not you may want to look into it.
Also, if the grip does feel a little large there are several excellent Glock customizers who offer a grip reduction service to fit your hand better.
Check to see which generation the gun is. The later generations are very easy to tell apart based on appearance only. The 3rd generation has finger grooves on the grip and an integral accessory rail under the barrel (for lights, lasers, etc.) The G21 that I picked up is what they call (un-officially) the 2.5 generation. It has the finger grooves on the grip, but no accessory rail. Either of these two generations are a steal at $400. 2nd generation and back are harder to guess without consulting the serial number. Each serial number begins with letters, which correspond to a manufactur date. The internet has several resources to find date/generation info based on these numbers.
I say buy it, shoot if, if you don't like it sell it for what you have in it. I'd give you your $400 back (after Christmas, of course
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