I have been diving for a long bit and I have to say, simplicity is your friend. I do know many here will say buy for what your eventual type of diving is. There is nothing wrong with that, but it doesn't do you much good beyond getting advice from strangers on the Internet (no offense guys).
You can get gear through three options really. 1) buy new on the advise of the shop, research or diving friend 2) buy used based on research and advice 3) Rent until you know what you want. A lot of new gear purchased ends up being bought by guys like me. Some student will get all kitted out, make 20 dive over a couple of years and never hit critical mass. All new gear will be pretty much good to go for the first couple of years. Beyond that, you will start to develop a critical eye that says "you know, I never really used that and it was $200..." or "should have spent more for such and such...".
I would recommend set a reasonable budget for what you would like to do and work down from there. Personal gear where fit is most important should be bought first. Mask, fins and snorkel will be used forever. Wetsuit and BCD follow pretty quickly. With the exception of the BCD I have always bought these new. Most BCDs are safe when new, but personal preferences are big because in the water the BCD is really the piece you will interact with most and there are a lot of different features and options. I will let others sling mud in that fight.
I have had very good luck buying used, but never done it through an online purchase (except a light, which I regretted). Things I look for in a used BCD is that it is fairly modern and look pretty new. (A good one will last forever and a cheap one will get beat up). I pay attention to whether it was stitched or glued (I avoid BCDs that are light weight material or have things that were glued when the could have been stitched). At the end of the day, you can get good stuff or not, depending on your knowledge and purse.
A really good regulator will last for decades and they aren't necessarily expensive new. I am letting my daughter use my original 1980 regulator next summer. If it is properly serviced it will work as well as new if you get one used, just pay of for the servicing of it before you dive it. Some brands have better reputations than others, Again I will the gear nerd argue it out (they love it too much to be denied). Dive computers I would buy new and realize that to start you are mostly interested in recording data on your dive (depth, time, temperature and surface interval). They can get over a thousand really easily, but you can find one that will keep you safe to start for less than $200.
Rental for a lot of gear might be good option if you don't know what you want or are just going to buy gear piece meal as funds become available. Some shops will have a try before you buy option either on a pool night or as rental gear. Do the dive shop a favor. If you are going to be buying online, don't suck up there time trying on gear and not letting them have shot at your business. Ask them if they will match prices from online. My shop is usually competitive on price when I ask. I usually ask if they'll do a cash discount, too. For the usually small difference in price, I' rather support them.
Remember, advice is free, so you get what you pay for.... And most of the gear you start with may not be what you want in five years, so don't go insane.
