Welcome to this forum , Itchy.
First start to u/w photography and you are off to DSLR, how lucky can one get
Regardless of which 6mp DSLR u get, the difference is going to be only SGD $1500 max. Tks to Canon and Nikon who came out with "entry level" DSLR, you can now get a 6mp DSLR body for SGD 2K. Housing, ports are all going to cost about the same for various DLSR models made by the same manufacturer - Ikelite, Aquatica, S&S, etc.....
I went thru the same dilema and the following are my considerations and deliberations before I decided on my current system : Fuji S2 pro in an Ikelite
housing.
1. Nikon vs Canon. I bet u never expected the question that sparks numerous wars in forums applies to u/w photography as well. I stick to Nikon not becos it is better, but becos most pros use Nikon, hence new u/w equipment always appear to be made for the Nikon system first. Also since there are more Nikon u/w photographers, second hand equipment is easier to come by. FYI all my gears are second hand, except the cam body and strobe arms. I save more than 30% in my setup cost.
2. Do you own any existing u/w phy equipment which u like to carry over? If it saves u money , then stick to the that system. However, do not be penny wise, pound foolish.
3. Back-up service. **** happens and it helps if there is a local service facility. S&S has the capability to service your problem locally, while most manufacturers are most glad to help if you get the housing / strobes etc sent to them. From my experience, Ikelite repair facility in USA provides first rate after sales service.
4. Strobe / Housing compatibility issues. For DSLR, I guess you are stuck with digital strobes - Ikelite DS 50 and DS125; S&S DX90 and DX25; Inon Z220. Make sure your housing choice has compatible syn cords for your strobes. For your info , Oceanbrite has "entry level" housings for D70 and 300D. But don't ask me anything else as I have not seen or use one physically.
5. Ports. Make sure your housing manufacturer has the ports for the lens you wanna use. Usually not a problem. Some use extension piece which means you can lengthen your port for longer lens. Some, one port for one lens. For example, I carry 3 ports for my WA, 60mm and 105mm for my Ikelite system. If I were to use S&S, I would need one dome, one flat port and and extender, instead of 2 flat ports.
so have fun building up your u/w phy rig
Eric