Grob 103s are still about $40-$45K in value. Especially since all of the L-13s were grounded.
I'll run some numbers, but I'm sure 300# in the front seat would exceed the forward CG limit. No opportunity to add ballast at the tail like the DG1000.
Same with a K-21.
We only have 400# useful load, and no 100# instructor to train you.
I've seen some 240 pounders fill up the front seat of a Grob 103, I can't imagine how a 6 foot 3, 300 pounder would fit.

BillT