Finally a topic I can chime in on. I have been skating since I was 6 years old so about 24 years now and I think I've literally tried them all. Ive stuck by the last pair I got for about 7 years now (K2s) I got lucky and found at the swap meet. Theyre kind of on the expensive side though...but I lucked out and bought em from some dude for $25 that didnt know any better I guess. Range of motion was important to me so I went with more of a hockey style skate (short top, lace up with traps for extra snugness.)
What I have noticed about the cheaper skates is they are very stiff hard plastic with no moving joints to allow your ankle to bend naturally. If you skate for long periods of time on these it can cause pain in your calfs. Though if your not accustomed to rollerblades you almost certainly will get soreness in your ankles & calfs. So as far as the actual boot goes, I prefer the flexability for more control.
Wheels however are a different story, the price doesnt really matter too much on these...or the bearings. If you are going to be skating on rouch surfaces like asphalt you will want a very dense wheel so that they dont just crumble right away. The density is located on the sides of the wheel 88a being densest and 72a being the softest. Each one has its own benefits. Softer wheels will provide a smoother feeling where you wont feel tripped up by small twigs or rocks...or cracks in the sidewalks. They will also allow more maneuverability and sharp turning. Hard wheels will allow for more speed because a dense wheel will roll faster with less surface resistance. Speed skaters will generally lean towards the upper 80 range, but someone who say plays hockey will go for the softer ranged wheels, because of the added level of control they provide. Bearings are rated by an ABEC rating Abec3-6-9 etc the higher the number the the more precise and resistant to wear the bearings are. I believe it can go up to like Abec12 or 16 but I forget...I use Abec6's because theyre cheaper.
Downside though is wheels can generally run you like $40 for a whole set so you will want to rotate your wheels when they start to wear, and you HAVE to keep your bearings clean or youll lose that smooth rolling feeling, and replace it with workin your ass off to not roll very far at all lol. Another thing I have noticed is new wheel sets really mess up your balance, even after rotating it changes the way the skate feels and how easy it is to maneuver. So if at first it feels a lil akward and stiff like you cant control much, wait a lil while skating on them, they will wear down to fit your individual skating style.
Wheel diameter is a mystery to me but I do stick to 80mm just because I can burn thru a set of wheels in about 2-3 weeks if I skate for long periods of time. Lastly the brakes on a rollerblade can sometimes be somewhat akward, personally I just remove the damn things cus they caused me more pain and grief than good. Just learned other ways to brake instead. Anyway hope that helps ya shop.