Some habitable worlds is fine (and good for producing food) but there are also plenty of worlds in BT that aren't very habitable at all, yet valuable for reasons like natural resource extraction. I think what would be more realistic is that a deep periphery realm has a mix of both. You've got a few rather pleasant worlds in there, and a few that are pretty hostile but you can't resist what they have to offer.
You'd want some of that. Some natural resources and raw materials to work with. At first you can just cultivate them and use them yourself or perhaps export them (or both) to build up some basic industry.
The eventual goal would be to build up some domestic industry. The ability to make your own 'mechs, vehicles, infantry weapons, fighters, jumpships, dropships. Even if it's a fairly limited range of items and you are still semi-reliant on imports, it's important to have something that offers the ability to build and repair/maintain those kind of assets. For simplicity's sake I only listed military relevant items, but you'd need a whole host of civilian industries as well for a lot of the same reasons.
Realistically, it could take generations to create that. It's easy to imagine a deep periphery power with a backstory in which they've trotted along in basic survival mode, importing everything for generations, numerous setbacks along the way (pirates, pestilence, crop failures, natural disasters, plagues, in-fighting etc.), until they hit a critical mass of population and industry (along with education) where the pace of development began to speed up.
It's also easy to imagine that it started as a whole bunch of individual, separate colonization efforts. That those 12 worlds or so were first inhabited through at least 12 independent efforts that weren't cooperating with each other much at all. By and large, that's how it has happened, most of the time through Battletech's history, including the early colonization stories of the Great Houses. Each world was somebody's pet project colonization effort in and of itself. Some were independent, some were sponsored. Only later did the people on those worlds begin to work together or join a growing multi-planet society like a Great House.
In the earliest days, it's easy to imagine these efforts even represent different colonization efforts on the same planet. Corporation A decides to set up a colony in those mountains to extract metals, everyone they send is a corporate employee and their famlies, some will become semi-permanent residents, others are completely transient. Colony Group B consists of expatriates from a Great House that pooled their resources together and set off on their own to build a new life for themselves and they land and build a colony about 600 kilometers from Corporation A. On a sparsely populated world with a handful of colonies on them, there's no instant "global society" it takes time to build that. Until then groups could exist with little or no contact with each other for some time.
The alternative to that is like a mass migration, not just people but also all the equipment and expertise needed to get everything off the ground in a year or 10. Which probably means the deep periphery power was really sponsored or owned/controlled by an existing party that put together a massive colonization mission. Along with sending people, they packaged together the necessities needed to establish a few shipyards/factories right away, and dispatched some military forces or mercenaries, giving the effort a tremendous head-start.
