The Scubapro Mk 16 is relatively inexpensive, offers good performance and performs very well in cold water. I would pair it with a G250 second stage to gain the benefit of a balanced second stage with a diver adjustable cracking effort, but it would also perform well with the more standard and non adjustable R390 second stage. If you go this route and do not mind a standard sized second stage, save some money and get an R190 second stage with it. Same internal parts and same performance as the R390 but for about $50 less. Another option would be the balanced but non-adjustable S550 second stage, but for another $30 or so you could have a G250.
The Scubapro Mk 2 Plus first stage is also an option that offers excellent cold water perforance, bullet proof reliability and super simple servicing all for minimal money. The downside is that its performance is more limited than a Mk 16 or Mk 25 and I would not plan to take it below 130 ft.