i used to sell welders and do techinical support unless you are doing alot of welding 5kg is fine i have made a whole chassis inc all the bits to go with it on 1 spool of 5kg
your not going to find a hobby rated unit that will feed a 15kg spool well
and it will be rusty before you finish it
keep a look out on local gumtree and ebay etc and somthing usualy pops up if you wait its well worth it
i found a real old sip 195 for £30 cleaned it up and treated it to a new euro torch and cables all for £35
yes its nothing fancy but i have got good welds all for £65
idealy i would have liked to find somthing a little better quality but at the price i was going to give it a go and flog it on for more than i paid for it if didnt make the grade (sadly it works quiet well and holds a full size bottle)
if you realy realy want new try looking at somthing swp 170c or Parweld XTM161C would be the cheapest thing i would recommend