I've seen those in person and I did NOT like them. I'd find a store and try the ones you linked before you buy them. Some of those block systems rattle and shift a lot. The Bowflex ones seem like the best (I know, they're overpriced too). People swear by the PowerBlock ones too.
I thought 50 was enough for me when I started too but I'm using 70's on lawnmowers now. I'll be needing more soon.
I guess I'm lucky. I have a workout room in my building. We have dumbbells from 3 to 70. Not bad.

I'd probably buy the Bowflex or PowerBlock ones if we moved to the burbs.
General question to all: I have a cold. I have a sore throat and I'm sure a running nose is right around the corner. Do I continue to push myself or just take a break? I think I'll try to stay on pace. I'm starting the 3rd month and I'm excited to see my progress. I'm just worried I'll feel fatigued and not get the most out of my workout.