Should I buy Van with a bad PCM?

It's a Honda Odyssey. 250,000 miles. The body is mint. Transmission rebuilt 2 years ago. New radiator and timing belt. New brakes and tires. Cold ac. The only thing is that the computer is going bad and it's causing the battery to drain overnight. The guy only wants 500 bucks for it. I think it's a good price and deal. New computer from dealer is about 1200 installed. I think it is worth it

I forgot to mention it's a year 2000 model

Probably nothing wrong with the computer. Could be a good deal but don't buy the computer, they don't usually cause a draw overnight.
They have problems with relays that get stuck and keep things like the A/C clutch engaged overnight.

You have a Parasitic Draw. You would need to do a Pinout to confirm that. I doubt that was done or that the ECM/ECU is the reason for the draw. With those miles the Ignition Switch would be suspect… Sending power to circuits when it shouldn't. But for that price it's a deal. Here's a better deal if your ECM is bad http://www.ebay.com/...sacat=6000

I doubt a PCM (ECM) with that big of a draw could function. Have the battery tested then do this simple test. You can buy a Volt/Ohm meter for under $10 at Harbor Freight or get a loaner from a parts store (you put down a deposit to borrow it you get your money back when it's returned)
http://honda-tech.com/tech-misc-15/resource-measuring-parasitic-current-draw-2893985/
You don't need to really be concerned with milliamps of draw an over night battery killing draw will be over a few amps.
You'd be surprised how many glovebox or underhood lamps are stuck on. And stuck relays plague these older Hondas.
If you feel like having a shop fix it. Just tell them the problem, battery dies when it sits overnight.don't tell them what to do, change the (fill in part name here) . You'll have no recourse when the problem isn't fixed.

I doubt the computer is draining the battery overnight. The wires running to the PCM would be melted. It's more likely that the alternator has a bad diode and that's causing the draw.

Parasitic electrical faults drain the battery overnight. They are usually easy to diagnose and inexpensive to fix. They are never in the PCM. That sounds like just what some shady mechanic told the owner to fish for a big moneymaker.

Yep, I would pay 500 bucks for a 2000 Honda Odyssey in the shape that one is - in a heartbeat.

Electrical fault is going to be in a component that should turn off with the key out, but doesn't. Something like a bad relay or solid-state device, like the radio amplifier, a headlight relay, a blown rectifier in the alternator, or something with degraded insulation resistance to ground. Once I find it, it can just disconnect that circuit when I park it, until I get it fixed.