This type of thing has been discussed on here in the past and you will find this information if you search the site. I could search for you, but I have other things on my mind at present. Use the search box to the top left of the site.
First things first though. You should start by telling us what you want to achieve.
What are you planning on doing work wise?
Popping the ROV into a 1m horizontal tunnel?
Seabed surveys?
Platform and pipeline inspection from a ship?
Working in harbours?
What kind of operating conditions do you anticipate (Depth, Weather, Currents Visibility – inshore, offshore)?