Hi, dellgonzales, welcome to Windows Central!
The L1520 is a flagship, one of the best-spec-ed (except for the camera, but the camera is still good) Windows Phone currently out there. That will make it future-proof slightly longer than its peers... but as with all devices it will soon be outdated.
At $300, I think it's a fair price, provided that it's brand new and has the usual 12-month warranty; that would be like having an extended warranty since the L1520 has been around for more than a year now.
If there's a phone that should be compatible with Windows 10, the L1520 should be the first one. Nonetheless, all Windows Phones should be compatible with Windows 10.
Then again, there's no real news about the releasing of a successor to the L1520 - the same size. My advice would be to try the phone out, if you could get your hands on a working demo unit. If you really like what you see, then get it. Chances of regretting should be minimal if that's a brand new flagship phone... just don't keep it in your jeans pocket since there are many reports of breaking screens from bending.