IMO 300 miles is really not a big deal.
If you can get the same deal for another 5 go ahead, if this one got everything you wish for go for it.
I got mine Cx5 03 km on it.
The thing that really helped convince me to buy from this particular dealer was that they offered free lifetime oil changes. I could have gotten a car from another dealer that had less miles, but none in my area had the same offer.