Buying equipment is tough, there are so many choices and no Consumer Reports to help you out.
But there are some general observations.
First, every manufacturers mid to high end regulators work extremely well, so there are no truly wrong choices. Narrow it down by price and any features you feel you need and then search this site and see if you can find it reviewed on scubadiving.com. This should give you a short list that you can ask about here.
BCs are trickier, at minimum you should try it on and you'd be a lot better off diving with one before you buy. There are three main types, standard jackets where the air cells are all around you, back inflates (looks like a jacket, but the air cell is only on your back) and backplate systems that use a metal plate as a place to attach the air cell, straps and everything else. Each of these feels different and fit is vital (less so for a backplate system since the straps are completely adjustable) so a trip to an LDS or scuba show is in order, unless you've rented/borrowed something that you really loved.
Drysuits are like BCs in that fit is vital, but they cost a lot more and if you aren't a stock size you have no real chance of trying them out. So you are going to have to trust your dive shop and/or the folks on this board. I just bought a drysuit based in large part on feedback here. I got it from a dive shop's website to save money, but hedged my bets by going with a custom suit (I had a tailor measure make the 20+ measurements required for a custom suit and faxed them in).
FWIW, my take on Poseidons is that they are impressively engineered, but needlessly expensive and their relative rarity could make minor problems a real pain to get fixed if you are on the road. Sort of the Porsche 911 of regs.
If you can live with a Honda Accord, for around $300 you can get a decent reg with octo from one of the reputable online dive shops that will do great for 99% of non-tech dives. If you ever do get into tech or that 1% of dives where you'd need a Poseidon or equivalent, at least you'll have a back-up set for your warm water vacations.
For the DIN versus yoke question, it depends on where and what kind of diving you'll be doing. Very broadly, most rental tanks in the US and pretty much all in the Carribbean use a yoke, while europeans and tech divers prefer DIN. Another thing to keep in mind is that yoke adapters are a suboptimal solution: something else to lose/break, makes it more likely you'll bang your head on your first stage. Unless you were absolutely sure you were going to be getting into tech diving or were going to do all your diving locally with your own tanks or could easily rent DIN tanks, I'd go with a yoke.
Finally, I'd be a little concerned about the motives of a dive shop owner that automatically directs divers with questions to the most expensive stuff in the shop.