Sean
Budget seems reasonable to get a perfectly satisfactory welder. Hard to know what the right price is with so many known brands coming up £50 or more off.
Main things I'd look for are a decent warranty from a known brand or supplier who is going to be around should you have problems. Inverters are intrinsically less rugged than buzz boxes. Overheating is the usual reason for early demise so look carefully at duty cycle. Specifications are very misleading. 50% duty cycle doesn't mean half the time welding and half the time waiting for it to cool down. Actual welding time will be rather less.
For example my Fronius specification is for the given duty cycle over 10 minutes. Then you have to wait until it cools down. Flat out at 140 amps 10 minute specification is 35 % duty cycle. Cooldown time if you do red line it is maybe 20 minutes before the watchdog circuit lets you start again. But the fan will still be going so its not cooled right down. Now thats a full on industrial welder rated 100% duty cycle at up to 100 amps and even at 120 amps you don't really have to think about duty cycle. Fan may be running permanently tho'. Had to think when I used it at 140. That sort of toughness and safety margin comes expensive.
Gotta accept that with a lower end, i.e. affordable, machine you need to be a bit more careful in use. Doesn't help that many suppliers inflate the actual capability of the machines with an unrealistically low duty cycle at maximum output. Doesn't help that there is no time specification either. Sheer mass of a hefty heat sink may give a minute or two flat out before it warms up enough to endanger the electronics but then it takes a long time to cool back down. Noodling around E-bay I see a few likely suspects for this trick. Nobody specifies cool down times.
20% duty cycle over 10 minutes is only a couple of minutes welding. Realistically anything under 40% duty cycle in the range of currents you tend to use means a serious risk of trespassing outside the safe operating area and into the will it / won't it go pop region. No one puts a stop watch on their welding time and I guarantee that how ever carful you intend to be you will overstep the mark at times.
If you possibly can find one with 100% duty cycle at the maximum current of the rods you will normally use. 80% will do for the upper end of currents but you will need a bit of care. Helps that inverter welders work fine with lower currents than buzzboxes on a similar rod size. No experience of them but the Draper 64533 INV146 seems to specify well with 40% duty cycle flat out.
Clive.