I consider 3mm to be around the crossing point of where I would use MIG or stick welder. If you weld thinner than this you really need to look at MIG, as stick welders will tend to burn through thinner material. If I was looking for a MIG at the cheaper end of the scale I would consider Clarke as they have a good reputation among DIY'ers. Don't forget the cost of getting a gas setup. If you tend to weld thicker metal I would look at an Arc welder. Personally I don't get on well with the cheap 'buzz boxes' but have a cheap inverter welder. These seem smoother to use but have a lot of delicate electronics so look at the warranty if you go this direction. The other alternative is a good second hand unit but make sure it works correctly. One step up from the DIY units you are looking at makes like Portamig, Oxford, R-Tec, Parweld, SWP etc.
Portamig and Oxford are the same make, out of the same factory, you just pay a hell of a lot more for Portamig.
You can get thinner rods (1.6mm) for thinner stuff but they're a pain to use and stick more than they strike.
For what Lastminute wants, better off with one of those cheaper inverters that Aldi/Lidl tend to have in. Apparently they're not bad for what they are (just bin the toy helmet that comes with it)
I have just purchased my first welder, there is a great mig welding forum based in the UK which is helpful, I ended up driving to a welding supplies shop and along with an hours training (he was quiet) ended up with a GYS Easymig 140 which will easily do 3mm on a 13 amp plug.
Although it can be quite addictive this welding lark, so a few tacks may turn in to more
for the odd tack here and there or just to have around for the odd repair it's well worth forgetting the gas and getting a rig that can be used with gassless wire.. no bottle fees and it can be used outside when it's windy.. I got rid of my bottle years ago and went gassless.
It saves money and I've welded the chassis on a few Landrovers and even built a whole new rear floor and tailgate on a Discovery without any problems.. coated wire is easy and cheap to get off ebay..
I have a snapon 130 mig but under the sticker it's just a cheap mig sold under a few different names.. keep an eye on the facebook market place and ebay local and they pop up quite a lot..
I bought a job lot of hobby migs (three in total) for £25 just to get the handset off one to replace the one on the snapon after I ran over it with my sons car.. they were all working so have been set aside for future use..
Not sure if it's any help but I needed to tack a few things too so got an electric Sealey MIG welder (this one) from Amazon and it's been fine. It's obviously not a high quality machine but for tacking and very occasional use it's been incredibly useful and for not much money.