I looked into this upgrade and what I ended up doing was to keep my intex 2100gph pump, invest in a bigger 18" Hayward sand filter and increase the flow by adding a Hayward through-the-wall skimmer and return. Mine is a 15' x 48" Prism, and the original fittings were 1.25". I cut a new hole for the skimmer and enlarged the return to fit the Hayward return. I preferred that return over the Intex 1.5" one because the Intex one actually narrows inside down to something like a 1" hole, defeating the purpose. It also required a bigger hole in the pool wall. I decided against upgrading the pump because a decent pump will require a 220V outlet, which I have not yet installed -- and I'm not certain I feel good about installing close to a framed pool. A whole thread on this from last year... Upgrading the filter was easy and not super expensive, and the effects are striking. It ran the entire season last year, with daily swimming by 2 - 4 people, and we never had to do anything besides regular maintenance. No shock, no SLAM, no algae. Disadvantage is that now you no longer have an Intex pool, so you will have to get really good at adaptors.
To answer your other question, I found that the legs on the side with the pump have started to rust after one season. So the next season, I sprayed the edges of all lefs and frames with Rustoleum. This year, there were still some traces of rust, and I dipped the ends in vaseline before assembling. We'll see next year. The damage was never extensive, and we have some fairly regular and copious rains, but the part that rusted was only the side of the equipment, both sides. Interestingly, you can purchase replacement legs.
Mine is a newer model with the plastic connectors rather than the pin system they used to have. That seems to leave less components exposed to the elements.