Stick with the "light" part and it's OK. Over the weight limit though and it gets complicated - you can't re-use car brakes, you need trailer brakes to get adequate efficiency. Read the regs (or the precis of them in the trailer seller's catalogues) before you start.
As it's hard to find suitable donor cars anyway these days, the easiest route is to buy a ready-assembled axle, with rubber trailer suspension units on it. These are also a better match to typical light trailer weight.
2" steel angle iron will weld with a cheap stick welder, you don't even need a MIG.Indespension, Towsure or any of the others sell plans and kits. Definitely get tehir cataogues first.
OTOH, Halfords et al. will sell you a cheap trailer that has low-maintenance aluminium bodywork and for not much more than the cost and hassle of building one.