If your after rats then it's going to be low light conditions, a zoom scope will allow you to drop the mag down and brighten the image, but then allow you to increase the mag and shoot farther when it suits you.
Mildot reticle so easy to see in low light and good build quality/lenses for the money - http://www.uttings.co.uk/p112248-bus.../#.WU1ipU2WyUk
Mine was a '3-9x42 AO/MD' and every time I adjusted either the mag or the AO or both the p.o.i moved and quite considerably as well, it used to 'bark' when I shot the rifle which I could only put down to the AO spring resonating and the reticle was like looking at 2 telegraph poles, eventually it started to go blurry every time I shot the rifle so it got binned...
I'm quite interested to know if the new stuff is any better but I'm not prepared to risk my hard earned on another pile of pants like the last one...
.22 S410...
.22 Webley Xocet...
.22 HW95k...
MTC Mambalite as then you can stick a WDV on the back at some point. One of he best ratting set up there is. The Mamba has the features for the NV digi add ons. Not the most fantastic day scope in the world but it does fine at air rifle normal ranges.
Most of my scopes are 3-9x40 on pretty good quality, I can't really justify buying top notch scopes but if I could afford it then I probably would, I've always been a stickler for good quality lenses and understand that if you start buying cheap big arse scopes with high mag and lots of pointless bits on it then you are normally going to get crappy lenses but if you stick to a scope with either a 3-9 or fixed 6 mag then you can get something half decent for about the same money, I recently obtained a secondhand Nikon 3-9x50 non AO which has excellent lenses and sight picture all for about £85, perfectly acceptable for the sort of shooting I'm doing.
Pete
Far too many rifles to list now, all mainly British but the odd pesky foreigner has snuck in
What you need is a Leupold VXiii 4.5-14 x 50 LR. Crystal clear in any conditions with side paralax for closer shots, and I happen to know where theres one available for considerably less than your max budget!
My ignore list: <Hidden information>
Biggest scope I have is my old tasco world class 3-9x50 which is great scope for lowlight and lamping and for that it's served me well. Certainly doesn't feel over sized or too big.