softer lead compounds like jsb / air arms tend to deform quite a bit and are usually accurate at longer ranges, if your only shooting rats at short ranges then cheaper flat head pellets work well
I just use a smear of the black moly grease from halfords on the spring and guide and a thinner smear of silicone grease on the piston washer works ok in my guns
would'nt bother with a ratcatcher I'm average size and I.m lucky to reach the trigger because the stock is so long and my neck needs to be about 4 inches longer to see through the scope
as you say its a bit pricey mate and there are a lot of other sub 12 airguns that can do the job just as well have a look in the sales section on here there are a few bargains to be had
I walk from my house to my allotment with my gun in the bag regularly when I'm after the rats or just a bit of practice as do a lot of guys can't see how they can make that work unless I sit in the house an shoot out the window
to be honest there will be no difference in light gathering between 40mm and 50mm lenses at low mag its just the 50 will have a slightly bigger field of view and I have both hawke and nikko scopes on springers and no problems with either