It probably is a good deal, with the one caveat of the batteries having probably sat in a shelf for thee years.. But I'm not entirely sure that is necessarily a negative with li-ion batteries.
You will not be able to use any 18v batteries on a T15 but the t15 batteries can be used on 18v machines, sometimes at a cost of slightly less rpm. Many many many satisdied customers here taking about the t15 over the years. I've never used a 15v festool so can't comment myself.
But for what it is worth, i found the C12 way underpowered for the size of drill it was, and that was 5/6 years ago. Despite a lot of reports here where peoole have apparentlyl been supprised and impressed by the power the c12 had.. But ive bought 3 T18s because it's my favorite drill ever by far (Although it does need the compact batteries for me to love it quite so much as I do) I have loved the cxs, but it's rare i ever considered being able to take it as my only drill for a day, as i so often need more power than it has.
I think that providing raw power is not a main priority, and you are OK with the partial compatability with the current 18v range, the T15 will undoubtedly be a wonderful drill and $300 is a great price.
It is worth noting too that the modern chargers for the T15 and all 18v rage are cross compatible. All your chargers will charge all your batteries (assuming you don't have any 8+ year old festool drills around, and excluding the CXS/TXS)
