We were looking at a slightly higher performance aeroplane (TB20) and the running costs of parking outdoors based at Biggin was around £16k just to have her sitting on the ground.
Which meant that for 4 of us I needed to fly 35 hours or more to be able to break even with renting, excluding the cost of the share, which is pretty good really. But then you may well get incidentals... Ie: replacing instruments, suction replacement...etc... Which will cost a bob or two!
The annual estimates were in line with the current owner' spend. If there is a decent AA5B for £16k - I would be surprised however! Check if it includes VAT, and the state of the aeroplane. I have rarely seen one for less than around £25k (and that's a 1970s with more hours on the clock than you state - look at the engine's age, will it need replacing next year? Compression ratios on last annual? Damage? )
The best advice I can give you however, is to talk to another plane owner at your local airfield. He will shed a lot of light on the buying process etc... Then never buy an aeroplane without having a second independent engineer' opinion if you can get one. It may show that ADs were not kept up / a lot of costs are coming up really soon, and will paint the picture a little clearer.