Marine aquarium setup

I am new to marine aquaria, and would like some basic help if possible.
I have at present two tropical aquaria, but would like to set-up a marine
aquarium.
I have been advised many different things by my two local aquarium centres.
Please can anyone advise me on the correct procedures to take.
I understand I need a quite large tank for marine life, however nobody has
clarified what large is. Can anybody give me an ideal in size or gallons
for the smallest tank which would be deemed suitable.
I understand that I then need heaters, lights and a protein skimmer. Is a
protein skimmer the only filtration I need? If not what else is needed,
biological filters? I see from looking at skimmers they vary alot in price
from £20ish to £150ish. I see the amount of water they filter varies but
what else should I look for?
Am I missing anything or is that all I need apart from water, salt, sand and
coral pre fish?
All advice and recommendations will be gratefully received. Plus any
recommendations of equipment or online shops for UK delivery is also
appreciated.

I am a total newbie, so please go easy on me.

Thanks