It sounds like the OP wants to build one from scratch, bringing in trailers as she gets more money. I do not know the zoning rules where you are, but you will have to start with a relatively flat piece of land with all utilities to the site. Then, after you get final zoning, you will have to install utilities to the individual sites. The building department could require you to do them all at once or in phases. The property will have to be fenced and you will need to pave the interior streets. If you have common improvements, such as a tot lot, picnic area, clubhouse or pool, those cost a lot. All those things will likely have to be done before you can move the first trailer onto the property.
Out here in the Bay Area, I can't recall any new parks since the 60's or 70's. Land is too expensive and the up front costs are too high. What we do see in the more rural suburbs is people placing trailers at the back of larger parcels and adding septic capacity (or in some cases hooking them up illegally to existing septic systems). For years the counties turned a blind eye to this, but now it would be close to impossible to get away with it.
Not sure exactly where you are, but you might be better off looking at an existing duplex in your price range as a start. The other possibility is to look for a small, failing mobile home park and see if you can get owner financing. Turning these around can be challenging, because it's so hard to get rid of the junk trailers and bring in better ones. Finally, if you own and the zoning allows it, look at adding a second unit to your property. If you can do a lot of the work yourself, that could produce some nice extra income.
