I used to own a Deere LX6, practically the same thing. $350 is a steal for a very good condition LX5. If you want it, and a 5' is about right for your 3720, I would jump on it.
FYI: although made by Deere, the LX series cutters are, IMHO, only middle range quality, not the equal of Woods or Bush Hog high end models. But well above King Kutter and other cheapies. I wouldn't pay a premium price for a VG used LX, but $350 is really inexpensive for it.