Buying used gear can potentially lead to some great savings but more often that not it results in headaches and unforeseen expenses.
I have been diving for 30 years (recreationally and professionally) and I am still cautious about buying used gear. Too often you are buying someone else's problems.
With regulators, you really have to know what you are looking at and evaluate the age, condition and parts availability (not to mention the performance). You should figure at least $150 on top of the purchase price for a full overhaul (assuming parts are available and you can find a shop that will service it) and possibly much more if non service parts are damaged/missing. Computers that don't fire up are suspect .... it could just be a $15 battery but could also be flooded or a bad depth sensor (especially Suunto).
With BCs, it's a bit easier to judge condition -- you can inflate and make sure it holds air -- but there are other considerations. What style are you looking at -- back inflate or a jacket; cold water or warm water travel; backplate and wing? Each has pros and cons depending on the diving you will do. On some older weight integrated BCs, the weight systems were pretty unreliable - do you know what to look for? Do you know how to judge the fit of a BC? A poorly fitting BC results in frustration and wasted money.
I would recommend going to your local dive shop and seeing what they offer in your budget range. You will no doubt spend more upfront than with used gear but if you deal with a competent sales person, you have a much higher chance of long term satisfaction, not to mention a warranty and service support down the road (possibly free parts for life).
One clue to finding a competent sales person .... they should ask you about the kind of diving you do or plan to do before they steer you toward any specific products.
If they can't meet your budget, you are still free to look for used gear but you will have a better idea of the type of gear you should be looking for.