MarshalN is the one to talk to about HK tea. He knows the scene very well and has tea connections I will never have! I am but a n00b, so I won't give you any suggestions.
For young sheng pu, you won't find as much of a selection in HK as you would on the mainland, and prices will most likely be much higher. HK is shu territory in a big way, and the range available is absolutely staggering. You can sometimes find good deals on bings if you look. Looking is definitely part of the fun for me as I sometimes find stuff I absolutely love (and other times won't buy again). Some tea stores will gladly sell you a leung
(tael) of loose tea.
It might be interesting to visit some of the better tea stores and try some of their house cakes since you won't find those as easily in China.
What kind of pu are you after?
You won't find a shark-like mentality at most stores in HK. Most places have prices clearly displayed and there is no haggling. This being HK, you might just get a sales pitch in English, although I don't think talk of emperors will be all that likely.
I almost always buy small amounts of tea from a variety of older (1950s-1960s) and lesser-known stores to see what each place has to offer and what their 'house flavor' is like. I quite enjoy the fact these establishments cater to long-term customers who look nothing like I do, which is entertaining for everyone concerned, and they often have great prices for great tea.
I tend to buy loose tea (more oolong than pu nowadays) the traditional way, however, and buy most of my bings on Taobao. The HK market is VERY diverse and you can get robbed or get astonishingly good deals on the same block, depending on where you go. Caveat emptor!