Paul, some claim truly reliable 'locking' castors exist, but I haven't seen any yet, either. My take is that even if the wheels do lock securely, there'll be some small amount of bounce in the rubber tyres, which makes for less pleasurable planing & hammering on it. There have been several threads on the topic of moveable benches, have a look here & here for some discussions & 'solutions'. The best of all possible worlds seems to be cam-operated castors which can be easily engaged & dis-engaged when you get the bench where you want it.
The weight of any bench depends on way too many factors to give an 'average'; overall size & dimensions of components being the most obvious of these. This small bench I made a while ago was conceived as something I could lift onto my regular bench to raise the working height when I needed it, but I got a bit carried away & added some extras to my original design. The top ended up weighing in at 45Kg, which at my age & stage of decrepitude is way too heavy to be lifting on & off a 900mm high bench on a regular basis! So it got its own undercarriage which takes the total weight to around 60Kg. That's adequate for occasional use, but a little light-on for an everyday bench, imo, so maybe think ~75Kg as a good starting point? You don't have to have all of the weight in the bench itself, an under-bench tray that you can chuck something heavy in, would help, & you can remove the ballast when you need to wheel it about, making that job a bit easier.
Cheers,