You won't find anything that sounds like a Mesa, except a Mesa.
Also, if you've grown accustomed to a real MkIV, a Line 6 emulation just won't cut it.
The thing to remember is that the whole Mesa Mark series are basically a hot rodded and revoiced blackface Fender Bassman....hence their surprisingly good cleans.
I'd like to be able to help, but I'm affraid the closest you'll come is one of Boogie's budget amps.....50 Cal, DC series, Studio .22, ect. Most of these are designed to be stripped down (ie, cheaper) versions of the full blown Marks. A friend of mine uses a DC10 and it's a great amp.
Also, depending on what they cost over there, the MkIII is in considerably less demand than the MkII and MkIV, and therefore is usually cheaper. They're just as good sounding as both the MkII and MkIV, but they lack in the "famous User" department, thus why they're in less demand.
I own a couple Mesas and a Laney GH100TI. I think the Laney sounds killer.....very natural/organic sounding high gain.....but it doesn't sound anything like a Mark.