I started with a 29g tank as my first and I agree it gives you a nice chance to get into the hobby without getting a huge tank. The bigger it is the easier it is with saltwater.
I had a hang on back filter, heater, hang onback skimmer ( not necessary if you are religious with your water changes imo
) and two powerheads, koralia's for water flow which is vital in a saltwater tank.
After about 40g I think a sump and drilled tank are your best options. At that point a skimmer becomes more important and you'd need larger flow powerheads of course for the larger size tank.
And of course a must have, especially for first timers is a test kit. The general saltwater API kit is a great start and run about $20 online.
And depending on if you want corals or not will dictate the lighting fixture. This is a huge amount of $$ compared to the other expenses as you need pretty good lighting to keep corals. If you are interested in fish only for now you can get any light you like.