Hello Bignol, welcome to WAB
In answer to your first question you don't need to add any soil or gravel to the bottom. All plants need to be planted in aquatic compost, put into pots and covered in pond friendly gravel; over time this will naturally leak onto the pond floor along with plant debris etc and soon make a natural bedding for the bottom. If you were to put anything at all in, just get small pot or two of sludge from the bottom of a local pond, this will have all sorts of lovely micro creatures in it to start it off, tho can have unwanted ones too !! I didn't use anything in mine.
Plant-wise you need to ensure you have :-
oxygenators eg myriophyllum - water milfoil and ceratophyllum - hornwort are good ones. Lots of your micro life will live in these. I also use pondweed but not everyone likes that.
deep water aquatics eg aponogeton distachyos - water hawthorn, a fantastic plant which flowers and smells and self seeds
marginals there are too many to name but I have acorus -sweet flag, calla plustris - bog arum, caltha - marsh marigold, gorgeous, myosotis - water forget me not amongst others.
oh and don't forget a nice water lily, nymphaea marliacea albida is the I have, great for a small to medium pond and the pads help keep the algae down in the summer.
A good book to start you off is the 'rock and water garden expert' by D G Hessayon, can get it off ebay for few pounds and I've recommended to lots here who love it.
Good luck and keep us posted with your progress
PS don't forget to ensure you have one sloping side so that creatures who fall in can get out and that includes froglets needing an exit after taddie stage. I used some pond-friendly pebbles and stones for mine.