Optima Yellow Top - Expected Life?

Hi,

Just considering buying a yellow top (D750) off a mate of mine. He
has had it about 4 years and used it for powering his stereo in his
car! Pyhsically it looks in top condition and un-abused.

Thought it would power my fridge and invertor nicely. Question is
what would be a fair price and how much longer could i expect it to
live, i.e. is it long in the tooth or do they go 'forever'?

Any experiences with them?

Thanks.
Jon