As an owner of several different Android mobiles, I'd really strongly say the Galaxy Ace is way down the list as far as those I'd recommend.
The two things going for it are (a) the size of the screen, if using it to watch some video clip {or playing a game} and (b) that it has a 5 Mp camera.
If you search the last year or so of 'Geeky Stuff' for 'Galaxy Ace' you will see a number of comments in the negative (because of lack of memory in this model). There's the Ace 2 phone and several others which are better. At the risk of boring other readers, I'll say there's the Sony Xperia Tipo (which has been sold at around 50 quid, new, in recent months) which has a load more memory and runs Android v4 (rather than v2.3).
Three has just (1st July) launched its PAYG '321' plan. 3p/ min for calls, 2p/ text, 1p/ MB. Now that's a bit expensive for data (compared with the unlimited options which start around 12/13 pounds for a SIM only deal) but generally much cheaper than other PAYG options from other networks... Best bit is that the credit doesn't run out until you use it up, it's not one of those 30 day nasties like most PAYG deals.
Alternatively, you might consider Ovivo Mobile (ovivomobile.com) where for 15 quid (one off payment) you get 150 minutes, 250 texts and some data {500 MB} each month
I have to admit to not yet having used Ovivo but plan to get one of their SIMs as I tied myself into a 24 month contract from Three (unlimited data, but only 100 minutes, plus a BlackBerry mobile and costing 13 quid a month)... and I think the extra 150 minutes a month for a very low cost is worth having. I have oodles of texts and data on Three but no easy / cheap way to trade those off for more 'talk'...
If I had known what Three were planning in July {their One Plan dropped in price from 25 to 15 pounds} I'd have gone for that, with 2000 minutes, 5000 texts and so on, instead of getting myself a BlackBerry phone, but it was either unlimited data and 200 minutes for 12.90 /month or 100 minutes, unlimited data and a phone, for 13.00 /month, so I opted for that!