I think you will be just fine with the
L3400. I have one with 30 hours on it now and I am mighty pleased with it. Mine has the HST.
I have 15 acres of pasture also. I went with a 5ft Woods BB60 mower but I regret not getting the 6ft model... the tractor could handle it just fine.
So long as you are not planning on pulling a baler you don't NEED to move up to a higher hp model for your needs. I think that the
L3400 will do just fine. I don't think that 15 acres is that much to handle with a 6ft mower - heck I'm doing it with a 5ft. I've bush-hogged at least 5 times this year as I have no horses yet - and loved every minute of it!.
Now, that said, I think if I were not looking for an HST, I would move up to a
L4400. I think it is a no brainer. It's not that much more money and you get a LOT more tractor - extra hp, better hydraulics and lift, more weight.
That's what I would do if I were you. I think that the
L3400 will do everything you have listed - it will be just right. The
L4400 will do it faster and is a heck of a deal if you have already decided you don't need the HST.
Good luck.