As to filling a pre-charged pneumatic (PCP) rifle, the cheapest way is to use a hand pump. This may be a physical challenge to you, but only you can answer that. Various size bottles can be had for filling from. Be aware that's going to involve paying a local shop to fill. For about the same price (give or take) you can get a small compressor. The thing to be aware of is to make sure it has a high-pressure moisture filter. The cheap ones don't and the little add-on filters still let some moisture into your gun!

Regarding:
1) Does a longer barrel versus carbine or bullpup give greater muzzle velocity ie hitting power?
1a) generally speaking, yes. But, the difference has more to do with the action's design (valves, springs, etc.) than with barrel length.

2) Same question for accuracy (up to 40 yards)?
2a) Accuracy is often times more a function of barrel design and ammo. Each gun is going to have ammo that it just won't "like" and other ammo that all go through the same hole. Best to watch some YouTube reviews like "Airgun Exploration & Advancement Channel" (https://www.youtube.com/user/svscialli)

3) Is .177 or .22 better accuracy over 40 yards?
3a) Again, that depends (sorry)

4) Which is better for rats and squirrels?
4a) Whatever you can consistently put hits inside of a 1" (25mm) circle at the expected distance.

5) What spec of scope would suit anticipated range? (metal not plastic)
5a) Not sure what you mean by "spec". I do pesting at local farms and ranches and rarely need to shoot anything further than 60-80 yards. The most magnification I use for this is 16x.

6) Is it worth paying for parallax free/ second focal plane?
6a) Parallax is a very deep subject. Let me just say this - I only buy First Focal Plane (FFP) because I depend on the things being correct at every magnification level. Second Focal Plane (SFP) scopes are only correct at 1 setting, which should be in the specs and on the box/manual.

Scope advice: Watch some videos by Joe "Cyclops" Rhea (https://www.airgun101.com/video-channels/cyclops-videos)

Bonus advice: spend double on the optics to get the most out of your gun! If your budget is 1,000 that means your gun should cost approximately 333 and the scope 666. Now take those numbers down until you either fit all the accessories into your budget OR you realize that you need to spend more money!

~ Mondo