Intro:
Hello Mark in Poole.
Building a pond is easy, dig a hole shove in a liner, put water in it job done.
If only it was that easy. A pond when built correctly is only built once, so don't rush into it, do some more research and if you can go and have a look at some ponds.......
Regarding what you have so far said:
At 3.5 feet deep your pond is reasonable but bear in mind that the excavation will need to be deeper to allow for the base.
You can use the paving slabs if you smash them up and use them as hard core.
A sunken drain (Known as a bottom drain) is there to collect fish waste, this fish waste then goes to your filter system - usually to a vortex chamber (Giant funnel with high sides) and of course, the bottom of the pond must slope to the centre where the bottom drain is.
You don't have to have a bottom drain, but if you don't its more work since you have to clean the pond floor (You really can get pond vacuum cleaners that will do this, but the best are not cheap, and they need to be pushed around like when you vacuum a carpet)
Fish:
I feel I should point out that your pond will be fine "for some gold fish" but too shallow for KOI, and KOI are expensive fish to look after. (Even a well looked after goldfish can live 15+ years)
Wall coating:
Some years ago now, mate of mine got hold of some "water tank paint" (As used in commercial water storage tanks that hold drinking water) he painted his newly dried out rendered pond walls (Two coats over several days) filled with water and dechlorinator water tested a ok, in went the fish, less than a week later all the fish were dead. Yes, the water tank paint was genuine, and any water stored was fit for human consumption, but that was the problem, it was safe for humans, not fish.
So he spent the next couple of weeks getting it sandblasted off and he had the whole pond fibre glassed.
You can get a fish friendly concrete sealing paint, its called g4 (Available in black, blue or clear)
Filtration:
Your filtration need to be able to cope with "fish waste" of any fish you have, and bear in mind happy fish will breed. Most (not all) people have a veggie filter as well as the "normal" filtration. It may be something you want to consider.
UV
You are probably not yet aware of such a thing, but in short it keeps your pond water from turning green (pea soup) brand is your choice, but size depends on flow rate to your filters.
Pump(s)
You need a pump big enough to supply the filtration system, in short you can turn a big pump down, but you can't turn a small pump up. And two pumps are always better than one.
Electricity
You will need an electricity supply for the pumps uv, lights etc. You can get "dubious kits" that enable you to DIY, but I would at least consult an electrician, and what ever you think you will need, you will always want more.
Thoughts
Doing it all your self, will take a lot of time, but when its finished you can stand back and say "I did that" and be proud. Do several drawings, sit and look at them, mark out the area in your garden with a rope or hose and look at it for a week or so, look at it from all angles, you can easily move a hose or rope now, you can't easily move a hole. And don't build it near any trees
And that's just for starters.
There are a few of us on here who can try and help, the above is how I see it.