First question, do you want to weld ally with it?
I have a GYS with all the features you mention, 160A with a reasonable duty cycle, better than that sealey cobblers. although its DC only so no ally
. AC machines are much more expensive than their straight DC counterparts. this model cost £400 some time ago, whereas a good AC model weighs in at £1000+!!!
Also, mine has an arc mode, which pushes mains current concumption to 32A single phase!! although tig mode is more efficient drawing 16A continuous and 20A bursts [hf start]. it IS possible to butcher a regular domestic 13a plug on, it wont't try to overheat until you set it on max power, although i take no responsibility for melting the spur
the features you mention should be available for £350-£500 new depending on item quality; the rule with all welders is to look at its duty cycle at max rated output
http://www.gys.fr/pdf/datasheet/uk/015647.pdf
good kit that comes with all but laminar flow pieces
or the ultimate:
http://products.esab.com/Templates/T085.asp?id=55552