First of all, If that's their idea of 'good', then I'd be warey of the fact of whether telling the truth about the firmware it's on or not. Quite often people will list PS3s as being on 3.55, when they're actually already jailbroken and on a much newer custom firmware - they just list them as being on 3.55 because it's probably a more common search term, meaning their add will get more hits.
Either way, it doesn't make a difference. As long as it's on either 3.55 OFW, or any CFW, you'll have the option to install any custom firmware that you wish to use.
However, the condition is absolutely dreadful. Being in that physical condition, I'd bet a lot of money on the console being dropped and abused in the past, and possibly even disassembled to reflow in the past, meaning the life left in it - if any - is highly questionable.
I'd keep looking around. I got an excellent condition CECHP03 console a while ago for £13.99, purely because the owner had the 3.56 hard drive bug, meaning it hadn't been used since the days of 3.56 which made me think it would be in good working order, which it was! All it took was a bit of work with my E3 Flasher, and it's now running happilly on 4.70 CFW.
Also, I believe £13.99 is more or less $20. I should think that if you look around a little more, you'll find something a lot better for potentially less money. Good luck OP.