You can certainly start with one strobe and then get a second when you are ready. The technique you use with one is slightly different compared to two - but it's all part of a good learning curve.
As regards which strobes to buy, it's worth doing a bit of a search on strobes here on Wetpixel. I think you'll find that Inon strobes are very popular especially the Z240 model - which was discontinued recently. But you will see people selling them in the Classifieds. The new model is the Z330 but hard to get at the moment. Check out too views on Sea and Sea strobes.
Whilst you may find you change camera and housing from time to time, if you pick the right strobes, they can move with you as you possibly change system. So worth picking them carefully and getting something good.
Likewise arms and clamps which you will need to hold the strobes. You will likely only buy one set as they last for years (I've had mine for almost 20 years) so buy well! Check out views on ULCS arms.....