Obligatory comment about still not having an HPA sub-forum.
HPA is 'high pressure air' - some scuba shops will charge you £15 for a fill, others just £3. Either way it's pretty cheap and you can buy a pump and do it yourself if you'd like. Another option is to buy a fully-blown scuba tank and then use a fill-station to syphon off air to your 'game tank' at home.
I'd buy a decent regulator because those things will stay with you until you sell them - the Redline Firebase or SFR (SFR if you can) are excellent regulators. The Ninja LPR is just a clone of the Firebase - I'd avoid simply because I know the Redline customer support to be absolutely stellar. Cost is up to £125 for an SFR with line. Tippmann is internally regulated, but I'm an idiot and forget this all the goddamn time.
As for a tank: Buy as good as you can afford because, as above, these things will go for 20 years (provided you get them tested every 5) and having extra volume is good on something like the Tippmann that eats more air than a solenoid-based engine like Polarstar, Wolverine etc. If you can find one, decent sized (40-60ci/cubic inch) 4500psi tanks can be found for under £80. If not, go for a 3000psi tank (again around 50-60ci is where you want to be for something like a Tippmann). Usually these run between £40 to £60.
Some shops do 'kits' for the Tippmann M4 that include everything to get you started, but I've found these to include cheap tanks with cheap tank regulators (the 'top' of the bottle) and also cheap regulators themselves. I would again really encourage buying the best you can as far as a tank and regulator is concerned as this will hold their value well and can be swapped between many different set ups.
The main issue with HPA is the user: It's something you should make yourself familiar with over time. I know far too many people running HPA setups who don't even know how to properly lube the seals on their regulator, or that you can't use certain LPR regulators on certain tanks etc. - you need to properly maintain your Tippmann too. It's a budget system for entry-level pneumatic blow-back, and whilst it works pretty well there are a plethora of issues that you can have if you don't properly take care of it. Do not just take what you'll get from this thread as all the information you'll need - I would recommend some Facebook pages, but most have been shut down. The Tippmann group is still alive and kicking though.