If you don't have a set of sockets, you need a set. Buying single sockets for the standard sizes does not make economic sense. Get a set.
That brand is not one we are familiar with in the States, but Google tells me that it is
very expensive for a 3/8" drive beam torque wrench ($225 - $245). It is thus likely that it is very good quality, but I know here very decent ones go for between $50 and $100, with cheap ass ones for under $25. If you have lots of tools and want a really good torque wrench, that's probably great, but if you have few tools and a limited budget, I'd spend at least 1/2 that on some other tools. That one may be a bit overkill for your needs. Professional tools have those kind of prices, and the biggest difference between them and decent quality mid-line tools is that the pro tools are made to work forever. The average bike owner doesn't need a tool designed to be able to work reliably 8 - 10 hours a day for the next 20 years.
Edit: I just looked at a picture of that torque wrench. That is a very fancy version of a "beam" torque wrench, and is not what we generally mean here when we say "beam torque wrench.
Your Warren and Brown:
What we mean when we say "beam torque wrench":
Is the Warren and Brown better? I have no doubt.
Do you need that level of precision? I very much doubt that. Oh, and the W&B appears to only work in one direction, while the standard beam torque wrench works in both.
, hi to all 
. can anyone tell me what the size's are of the cylinder and cylinder head " nuts" ? . have to buy a torque wrench and socket to suit, have picked out the torque wrench which is a warren and brown # 321500 ( any comments on this ? )but not sure of what size socket i need. the bike is a 1975 R90s.