OK, now I'm hooked, just got my 1st Ruger MkII last week (10" SS Bull) and I'm already looking for another...

I have found a MK II Competition Target at a gunshop, problem is that someone cut the barrel down to about 4", very professionally with a nice crown.

Looks to be in good condition otherwise and comes with Laser grips (switch on side panel, not sure if it's a crimson trace) but no sights.
Couple questions,
How difficult is it to change barrels (not entire upper, just the barrel) and how do you line it up properly? I've done Savages before, nothing that needed alignment though.
What's the difference between the Target and Comp Target models?
How much would be reasonable $$ to put into this deal, with laser grips, no sights and a chopped down barrel?
Thanks Folks, and thanks for talking me out of cutting my 10" barrel. I posted this on Rimfire Central as well.