When you decide to start (now or later, whichever) go for the seat attachments and a sturdy step stool, rather than a separate lil potty chair. It is SO much easier to keep things sanitary, and getting used to "going" places other than home is much easier, too. Unless you plan on them using separate toilets the rest of their lives, there's no need to buy two, lol (unless you want the convenience of having them in 2 different bathrooms in your home).
As for whether or not to start training now, there are as many opinions on this as there are women out there. I personally, would wait a bit, as basically YOU will be the one doing everything (removing clothing, putting him on the seat, wiping, refastening clothes, washing his hands, etc). Once he is a little older, he will be able to do many of these things for himself and that is the end goal: for him to go to the potty, without your assistance. If you start now, it may be a very long time before he is to that point. Are you willing to do ALL of it for that long? If you want to put him on the potty first thing in the morning and immediately after naps to save yourself some $ on diapers, go for it. But I wouldn't expect to be successful with true potty training beyond that, at this stage of the game. Keep in mind, if your kids were born at 29 weeks, their "true developmental age" is probably closer to 15 months, not 17 months, and that is WAY ahead of the "curve" for potty training with most kids.
My son did a great job training and was fully trained sometime between 30 and 36 months. My daughter was even better and was fully trained right about her 2nd birthday. By a lot of people's ideas they trained "early". Usually girls train a little earlier than boys. IMO, I would wait.