The lathe cost is the same for the PM single and 3 phase machines, on the 1440GT the single phase is 2Hp and the three phase is 3Hp. Either way, I recommend 3 phase in particular if you want to add a VFD. The difference in performance, features and surface finish is not trivial. Lathes are a bit different when it comes to VFD installs and how to integrate them into the machine. I am familiar with the Lenze SMV VFDs, but the programing and wiring has been worked out for the Hitachi WJ200. Matt sells the WJ200 and it is about $80 less expensive than something like the Lenze ESV222N02YXB, but I can help you either way. As previously mentioned, there is a very simple conversion process where you use the motor forward/reverse contactor to trigger the respective VFD inputs. This works well if the contactors are new and will act just like the stock machine with variable speed and controlled acceleration and braking. The one caveat is the foot brake, you can either use the mechanical brake but should add a dual pole limit switch, the second pole issues a coast command to the VFD, or the limit switch can be used to electronically stop the lathe. Normally you may not have both operational because the VFD has a programed deceleration which can fight the mechanical. I do have full system designs, this is what jbolt used, but it can be a bit overwhelming to start with.
It is a tough call between the three machines, the 1440's seem to be what you want. The 1440GT being about 25% more expensive with shipping. The 1440GS really has everything you want, but it will not have the user feel of the Taiwanese machines. It really will come down to how much more that is worth, and only you can determine that. All of them will be a huge step up from what you had. You will also need to factor in the additional tooling costs which can be significant. You might omit the preferred packaged and go a la carte on the accessories. I am a big advocate of the set-true type of chucks if you do any kind of repeat work or consider a high quality 4J independent or combo. Matt does sell the Taiwanese Chandox set-true chucks, which are less expensive than the Bison equivalent. You may also want to stock up on holders and few other items that you may want to package.
Mark