I would say it is easier to fit the ARB with the subframe off the car, I tried it once when it was just dropped a little and it was a nightmare. Mainly the inner bushes!
If I were you, I'd build up a spare subby with wishbones attached, poly bush the lot, new ball joints etc then swap the lot over in 1 go, easy. If you can go for an uprated one, a Golf G60/Corrado 28mm bar is a nice oem-look upgrade. These were also fitted to some SEAT ibizas

Oh and a new rear engine mount & gearbox mount too, go for the morte expensive 16v/G60 rear mount, much stronger!
For the rear ARB, you could just buy an aftermarket one that bolts underneath the beam, should match the 28mm front nicely.
To fit it you need to jack up the front and put it on axle stands, remove the front wheels, unbolt the rear engine & gearbox mounts then support the engine, undo the ball joints from the hubs, then 4 bolts hold the subby & wishbones to the car, once its lowered a tad you'll be able to get to the rack nuts more easily, once off just tie the rack up to something. Be VERY carefull undoing the 4 subframe bolts though, the captive nuts can brake loose which is not fun as big holes need to be cut into the bodywork to fix.