Now for the whole speed thing is a pain in the arse, because well I can't think of an example that shows anyone is DBZ going at sped of light but they are by no means slow either, and there really isn't any evidence to say how fast they can actually go, so how we can compare Super Buu to any character with super speed is quite beyond me.
As for strength that is a little easier, if you remember before the Buu saga Goku was training with weights of 10 tons and for him this was easy, when they were increased to 40 tons he couldn't move and had to go super saiyan, and he commented on how he couldn't feel them.
So for arguments sake Goku in his base form can lift about 30-35 tons.
Now in DBZ budokai tenkaichi 2 it was stated in a bio that super saiyan increase a saiyan's power by 50.
So if we take Goku's 35 tons and multiply it by 50 we get 1750 tons
Now arguably Super Saiyan 2 multiplies a super saiyan's power level by 50 again so that would allow him to lift 87500 tons as a super saiyan 2, and again if a Super saiyan 2 strength is multiplied by 50 we get 4375000 tons.
So I can estimate that as a super Saiyan 3 Goku lifts 4375000 tons
now if we look at Super Buu he was slightly stronger than super saiyan 3 Goku, by enough to not really be effected by him so we can say he lifts what? about 5000000 tons? now piccolo you could say lifts 35 tons, Goten and Trunks in the base forms 30 each, and Mystic Gohan well he was roughly double Goku's Power as a super saiyan 3? so 8750000 tons for Gohan.
add these together and you get 13750095 tons. So Super Buu with Gohan and the others absorbed can be estimated to lift this.
I know this is all speculation really but this is all we can really get and compare to Marvel and DC, now I don't know a great deal about Thanos but would he be able to match this strength do you think?
As for if the chi blasts travel at light speed, I would have to say some do and some don't, Piccolo is the only 1 with evidence to prove they can, but Kuririn for example fired a very slow Chi blast which was commented on in the Saiyan saga, so it varies, but I would say chi blasts can generally move at light speeds, Destructo discs and others I would think move quite slow.
This is why I find debating DBZ characters with comic characters such a pain cause they are so hard to compare.
and as for strength feats, I think Saiyan's and the main bad guys are the only ones who actually sit in the class 100 slots, if you remember in the android saga, Tenshinhan and Kuririn were trying to get into Gero's lab but could knock the door down with brute strength, it took Vegeta using a chi attack to do it (proving Chi attacks are alot stronger than their punches) (this happened in both Manga and anime as well) Then in the anime a super saiyan 2 isn't shown to be as strong as a comic class 100 cause Goku struggled pushing a mountain apart, and even to go as far as a Super Saiyan 4 struggled to push a building upright...so as you can see, such a bloody pain DBZ great story lousy consistency.