You don't seem to want to listen to others on here. We're not tryint to put you down, just being honest and realistic. A 156 of that age and mileage is not worth £900. Just look in the classifieds. There are far newer and higher spec cars than yours for less than that.
I sold my 2001 (Y) 156 2.0TS Veloce for £1200 over 2 years ago! It had 101k, full history with every receipt from new, one owner apart from me, mint body, recent belts and recently had all new wishbones, brakes, radiator, tyres etc.
That was pretty much the going rate even back then!
Old Alfa's just aren't worth anything. £600 if you're lucky.
I have a 2000 146Ti, great condition, full history... It's worth a lot to me because it's never let me down, but in reality I'd be better breaking it if I wanted to sell and and make the most I can out of it.
Spider values are very over inflated just because it's a convertible. The GTV is far better value.
If I was you I'd save the extra and get a good Spider, or just keep your 156 as it sounds like a good car. A Spider for that money will be a shed and cost you a fortune, and you will regret the day you let your 156 go!
Yes, you could get an mot'd Spider for £1000 but it will need atleast that again spending on it even if you do it all yourself... unless you can find one with a snapped cambelt and can get hold a cheap engine...