Any ideas on what the cost would be to develop a small mobile home park (5 acres with probably 25 units max)
Engineering?
Road cost per foot (hard surfaced gravel )
Fire water cost per foot(main at front of property?
Electrical feed per unit (120V 100 amp)
Water supply per foot
Septic per unit?

I'm actually looking into the same exact thing, almost identical size, but set up for a niche travelling construction/trades worker market. Similar concept to corporate apartments or monthly hotel suites. Manufactured homes for extended rentals or open lots for travel trailers of crews.

What's your approach? A. Developing just the land for lot rentals only? (where people bring in their own homes and just pay lot rent) or B. Development and also purchasing/setting up the homes for rentals? (where the park owner owns the homes too and rents them out like a multi-family property development)

From my research, it is very hard to be profitable on a new MHP development due to the high costs of the development...in the beginning. Key points:

Development costs, zoning issues, financing issues make it hard to start from scratch.....unless you have the perfect spot and a really good plan in place.
Profitability is very slow....this may slowly change as the park becomes full manufactured homes. This may take a while...a long while.
Until lot rentals (or home rentals) hit that magic # for positive cash flow, you'll be losing money.
If you can't fill the lots with people moving in their homes to your location , then you have to buy homes and set up for rentals. That changes the game big time.
Financing a new park build isn't easy either...really difficult. Lenders like stabilized properties to lend money on because of risk of failure.
Water and Sewer expenses can be a burden to install and upkeep if private and no access to municipal utilities.
If you want to get out, who' going to buy it if not at 80% capacity at least?...so exit strategy is essential.
But long term investing goals can work past that with the right plan and the right lender.
Also you can depreciate the land assets (road, facilities, utilities, etc...) over 15 years I believe, which is accelerated from traditional structure construction.
Once past the point of losing money, there can be good ROI.

Most "experts" in real estate investing will say that buying existing MHPs is much better as an investment. They say AVOID building new. A little value add and repairs on an existing park can often generate increases in rents to add more profitability. Once at profit generating operating capacity, there can be really good ROI on parks on both lots only or lots+homes approaches. Getting there without losing your shirt or goign bankrupt is the key.

I'd prefer to rent lots and expand in groups of 5 or so to minimize upfront costs. If I owned the homes, the goal would likely be to develop to a point that produced a good cap rate and sell.

Any ideas on good places to research costs of development?

I edited my post above to add some other key points.
Read this article and explore this site, MHU. One of the leading MHP investors out there.
https://www.mobilehomeuniversity.com/articles/why-you-dont-want-to-build-a-mobile-home-park.php

Do you already own the land? Doing it little by little may work better, especially if you have money to do it without having to go through traditional or hard money lending.

On 5 acres, without knowing the topo, it would be very difficult. We have developed subdivisions and it is a huge upfront cost. You will most likely not get a feasible bank loan, regardless of wealth, profit margin, really any factor. Banks don't want the land as collateral, like they use to. So if you don't have the cash, you better be looking for private investors.

I have no idea what it cost to put a mobile home park in, I can tell you what it would cost to put 25 home building lots on the ground. But dirt price is a factor. Road construction might me less depending on the requirements of the municipality. Do you have to put in a turn lane into the park, that depends on traffic count. That's about $250,000 to start

A good engineer should do a feasibility study to tell you everything you need to know, probably for a few thousand.

Buy some land, put down some gravel...plant a Bama flag on a pole.

Sit back and wait....

Wouldn’t be long

2 million should do it.

Bout right.

How about doing the 2 or 4 closest spots to the paved road first and see how they go before going all in? If they fill and work out then start adding a couple more at a time? I like the idea but not sure id go all in until I knew it would work out.

Bama on that brain...

Add 5 grand per lot for the pad and utilities to the cost of the land and the road system. Sewage whether hook up fee or septic tank is close to half that. I looked into it. I don’t want to be fixing raggedly mobile homes so I was just looking at lots. Problem is the market for lot rent in Bibb county is only about 125 to 150$. When you factor in maintaining common areas and dealing with renters it was too much investment for not enough return.

That would seem to be my thinking.
Where land is more affordable(rural), the lot rent is so low that it would take forever to pay off the initial investment.
Where land is more expensive(urban/suburban), the initial purchase price of the property would make it prohibitive from the get-go.

That's absolutely the very last thing I would consider on my land.
I'd rather have a reptile farm, run by a breeding pair of math heads.