Welcome to the forum,
You should get your monies worth out of it. Laguna II's do not seem to be the most reliable cars buy with only 49000 it hould have plenty of life left in it.
Search the forum for advice about the changing the belt. From the thread I read it seems the engine may have to be removed to get acces to the belt, hence the high cost.
Weak points to watch for are
tricky to get at on the V6.
b) rear axle bushes - failed twice in 2 months on mine. c)track rods, track rod ends & ball joints.
d) tyre pressure sensors play up
e) keycards are unreliable.
There is plenty of good advice to deal with these problems on this site.
The window mechanism is fairly easy to do. Remove the door trim, them peel back the membrane. remove the trim on the outside of the door at the bottom of the window. The window is held onto the mech with 2 plactic clips pull these off to release the window. Lift the window up and out of the door. Put it somewhere safe. Remove the mech (4 bolts). Swap the motor over to the new mech. Reassemble everything you have removed.
When you 1st close the window it will only move in 1" spurts. Get it fully closed and then hold the window switch "up" for 5 seconds. This should reset it and then the window should 1-shot to open/close.
My 2001 1.6 Lag II has done 144k, mot due this week (fingers crossed). just replaced rear bushes and track rod ends.
Good luck, let us know how you get on.