Most of my Festools are used. Unlike used tools from other brands, they are just as good as new because they are built so tough. They keep on going, like the energizer bunny. I myself don't think there is any reason to shy away from used Festools.
I have the EHL 65. I don't use it that much, but it has always performed well when I needed it. It cuts effortlessly through hardwoods like Meranti, Merbau and Oak on it's deepest 4 mm setting.
I've also had the 850. It is without a doubt a much stronger machine. Also much heavier and bulkier. It is for the bigger jobs. When you need to cut wider than 65 mm, or use it with really hard woods.
So when deciding which planer would suit me best, I'd ask myself what I want to use it for exactly. I wouldn't look at what a certain tool would cost, but if it could do what I want it to do. If the cheaper model can do it, great, get the cheaper one. If it can't, get the more expensive one. Cost a bit more eventually, but you'll be much happier when you work with it, and probably more efficient so you'll save money on the long run.
If you're only planing doors, the 65 would be just fine. But if you want to do bigger stuff like construction beams, an 850 might be better. The OP mentioned the different cutter heads for the 850. If you want that, the 850 is the only choice. The 65 simply doesn't give you that functionality.
