Just getting started

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Have you been diving locally yet or gone through Open Water certification?  I love diving locally and definitely don't intend to squash any spirit or anything, but I always try to throw the disclaimer out there that it doesn't seem to be for everyone.  My amazing wife, for instance... doesn't like the cold water, the lower viz or the fact that there is "nothing to see" since we don't have a million colorful fish and coral everywhere.  ;D  Natural/man-made formations, shipwrecks, finding random junk and/or simply attempting to improve your skills on a dive just doesn't end up interesting everyone for some reason!  :o  If it turns out you don't care for local diving as much as expected and would rather limit your diving to more tropical areas, your gear requirements/desires can change significantly.  Back to the original question, though...

Generally, Open Water classes will require you to have your own mask, snorkel and fins and you can usually rent the rest if needed.  If you've taken OW and don't already own these, this is still where I'd start as others have mentioned.  Also as previously stated, try before you buy on everything possible.  I think I was a bit lucky in many of my early purchases (ie. I haven't had to completely replace much of anything), but my usual dive buddy and his wife have each gone through a few versions of many items already.  Talk to your avid diver friend and to anybody and everybody at the dive shop for advice.

All that being said... I would 2nd the opinion of buying your own wetsuit first thing after mask, snorkel, fins.  This is the one most likely piece of equipment that could give you someone else's rash, for instance!  Most dive shops try to rinse them out well, but there's still nothing better than knowing exactly where it has been.  It is also an item that will not require regular servicing from the dive shop, so your price is pretty fixed over time.  When buying your own suit you can always customize sizing a bit if needed, or get a different brand that fits you better than the brand(s) the dive shop sells.

Beyond that, for me, it basically came down to weighing (equipment cost + service cost over time - convenience discount) vs (rental cost over time).  The convenience discount is the non-monetary savings from not having to pick your gear up before your dives and dropping it off at the shop after your dives... and remember that often times one or the other isn't very convenient for you because the shop is closed!  (evenings, sundays, etc)  The only problem is that this convenience discount doesn't kick in much until you have everything you need, including tanks!  If you want to get really in-depth (or want to fudge the numbers further towards buying more equipment  ), you could be sure to add in the costs of paying for extra rental days due to not being able to get to the shop in time!