flat vs incline / decline

A11

Here it the classic Q. Couldn't find info on here (or rather I didn't try hard enough to look for the info - sorry guys if its been covered, but please do redirect me).

Anyway I'm a beginner and thought before committing totally I need to get a feel and plus I am poor at the mo recently quit my job to travel
and just got back.

So basically I borrowed my bros sissy looking york weights (the ones with a gold plastic covering) cause he's never used it and has left them at home. However I figured I might need a bench. Looking for 1 it seems that everywhere apart from midlands has something for sale for cheap on ebay. I can't afford to go over 100 miles to pick up a bench costing a penny to a tenner. the cheapest thing I could find was a situp bench from argos costing £20.

Now I can do incline and decline workouts on this but what about flat bench? is it necessary? is it worth it? can I get through all workouts on a situp or is it worth spending extra on a flat.

Buy Sit Up Bench. at Argos.co.uk - Your Online Shop for Weight lifting and exercise benches.

I saw on argos for £35 one that adjusts and looks as if it can be set to do incline/decline and flat

Buy Pro Power Folding Dumbbell Bench. at Argos.co.uk

what do you guys reckon is it worth me spending the extra or can I just go for the situp bench?

Cheers