325i I am considering buying has been parked over six months and now won't start. It has a new battery. What can I expect the problem to be?

have a load test done on the battery as it may have sulphated up the plates and now will not hold a charge.. A long time stationary and short runs do help the situation.

Can you ensure the car is in park?
Move lever to D and then back into Park and see what happens.
I have heard with this particular model, we have the environmentally unfriendly RX 330, that the batteries lack of charge may play havock with the electrics in these cars.
I religously charge my battery every six months with a smart charger so its always in peak, I also find a quick zap at the first cold snap of autumn / winter gives us trouble free motoring.

I had a similar problem a few months ago. Ready to get major engine work done, got fire, timing was right, getting gas.....
An experienced mechanic asked where the car had been parked and how long. I had parked it in grass, over a wet spot. The battery was dead too. He told me to park it over a warm spot for a few days and try a new battery. Did that, with the hood open, replaced the battery, and it fired right up. Something about being too damp and/or the voltage in the battery was too weak.. All I know is it worked, and I drive it every day and don't park in wet grassy spots.

Your sterter is fried, get new one

"It won't do anything" means that there are no dash lights, no buzzers, nothing?
Any car has very slight electrical drains when it is shut off. For one thing, the clock is running, but there are a number of other tiny loads as well. With a good battery, you should be able to park a car for six months to a year and still start it.
However, if you start the car several times without running it long enough to recharge the battery, or if the drain is larger than it should be due to electrical problems or aftermarket accessories, you can get a failure to start.
Unfortunately, maintanence-free batteries don't tolerate being run completely dead very well, and many owners have unhappily found that draining an older battery once or twice was the last straw that ended their battery's life.
I suggest that you borrow a charger or a set of jumper cables to start the car, and drive it 30 minutes or so to fully charge the battery. If you have more problems, get the battery load tested at a shop that sells batteries to see if it is still OK.

It's been sitting for six months, plugs probably have corrosion or rust on them. That and your fuel is probably turned to soup. Put in new plugs and a really heavy duty fuel system cleaner. You might consider changing all filters too - especially fuel filter.

put one of these batteries on charge and leave it for a good week to de sulphate the plates

I had similar prob - no rhyme nor reason as to when, where or why it wouldn't start. After two new starters the shop finally found a starter wire/cable that was corroded on the inside. Mech' could move it back and forth and it would / would not start. Changed cable six months ago - no problems since. Hope it helps.

You have a new battery and it cranks. Turning over is cranking. You are saying it won't start after sitting for six months? What is an old mechanic? Was the cable installed at home during the one year layoff? It sounds as though you failed to hold the old mechanic responsible for installing the correct cable or adjusting the one you have ,so it will start. You have a safety switch issue by the sound of it. Make sure you have the right cable and then find out how to install and if applicable,adjust properly

Drained the old gas from the gas tank and refilled it with new, also replaced the gas filter and charged up the battery.