I believe in the Apex regs as do quite a number of other divers, the other alternative is the MK25 Scuba Pro, but very expensive and outside your budget.
One consideration should be (in my opinion 4 what its worth) how much does servicing cost and where can it be carried out.
Poseidon I understand are very expensive to service, if its too costly then will you skim/miss year two/three servicing in which case you don't want the regs
Apex cost about £50 for 1st, 2nd and Oct servicing, O2 cleaning is £3 more.
The TX50 is better than my TX100 to breathe off, if you don't want a swivel turret then get the DS4 (same reg 1st stage) for about £70 and add your 2nd stages to that