Hello Thosmeyrs,
As this question comes up often, I have a cut and paste answer. Skip the boring part and go to the bottom.
Shortly, you will be swamped with advise about buying a regulator, mostly by people who own a brand and are very fond of it. One thing to keep in mind about regulators is that most are great at what they do. So how do you separate the wheat from the chaff? That won't be easy, but it will be fun and entertaining. Soon you will find out there are quite a few gurus on this board in each section. It's helpful to pick out a few of them and read lots of their post and you will be off to a very good start. In the regulator section I can highly recommend the post of (in no particular order) Awap, DA Aquamaster, Captain, Pescador775 and Luis H …there are lots of others but that will keep you busy for a while.
At any rate, you want a regulator brand that has a good track record for keeping parts available for each model. Of course unless you intend to DIY, you also want a regulator that can be serviced by your local dive shop and lots of locations in the world. The two best companies in both regards are Aqualung and Scubapro.
My recommendation for the average recreational diver, i.e. not freezing water, 150 feet or less, is a simple non balanced piston 1st stage such as the Scuba Pro MK 2 (or Aqualung Calypso, Mares R2, Sherwood Brut, Oceanic SP4 and a few others.) This simple design is usually inexpensive, nearly bullet proof, easy to get parts for and easy and inexpensive to service. It is also easy to DIY because it requires only a few o-rings and a seat. Yes, the unbalanced design will increase the breathing effort at low tank pressure, but I view this as a good thing especially for new or inexperienced divers. See this thread:
http://www.scubaboard.com/forums/regulators/87374-scuba-pro-r190-2.html
Additionally, if/when one wants to start more advanced diving where high concentrations of oxygen will be used this type regulator can be moved to that application.
For a second stage I like an adjustable and balanced unit. This provides the ability to have the regulator tuned to minimum inhalation resistance with the option of the diver increasing the resistance it if he/she finds the need to. Having said that, a straight forward classic downstream design with a simple to replace set of o-rings and button soft seat can be highly recommended as it is a tried and true design, parts are usually available and almost anyone can service them. It is a good idea to have the same manufactures 1st and 2nd stage to make getting service less complicated.
For more demanding diving and high end/higher performance (higher price) regulators see the following link for DA's excellent advice.
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http://www.scubaboard.com/forums/regulators/181745-advice-regulators.html>
Now, if you have read through my entire boring post or have done the sensible thing and just skipped down to this part, the best advice would be to contact someone who rebuilds regulators like DA Aquamaster and see if he has a rebuilt ready to sell.
Good luck,
Couv