I'd imagine the Tabard for Wyrmrest, and then running level 80 heroics would be a fast way to max your reputation.

If you have the funds, the Vial of the Sands is going to be the easiest drake to acquire.
Spend a little gold (more than a little), and there you go.
The mount looks neat, and it is fun to have, plus I get a bit of notoriety/attention from having five on them on my five box team..

If you've only got 300k, I wouldn't recommend 10x Vial of the Sands.
You'll definitely want to keep a fair amount of gold in reserve, as making gold is easier with a larger amount.
The vendor portion is 29k per mount (less with Goblins or the relevant faction) plus 12x Truegold plus 20x Flask A and 20x Flask B.
That would likely put you close to broke.
There is a balance between not being able to buy upgrades, so having more gold then you can spend... versus not having enough to buy the next toy, whether that is Alchemy x10 (or another skill) or a 4.2 crafted weapon that is a significant up on what you have, etc...

My Vial of the Sands cost me 150k in vendor mats + combine fees; the Truegold was essentially free, but 60 bars could have sold for at least 30k at the time of the combine, so in all it probably cost 185-190k for five.
I had a little over 400k after the purchase, and going for a PvP focus this expansion... there was essentially nothing that gold could buy.
If I only had 250k, I wouldn't have made the purchase.