Deren,
Great questions about Habitat. I'm a board member of the Habitat for Humanity(Illinois) State Support Organization. Hopefully I can shed some light on your questions. Each Habitat Affiliate operates a little differently but let me share my experiences from working with our local affiliates.
1-Land Donation-Land to build on might be acquired by donation from individuals, local city/county or might be bought on the open market.
Each local Habitat’s family selection committee selects homeowners based on three criteria: The applicant’s level of need, their willingness to partner with Habitat, their ability to repay a mortgage through an affordable payment plan.
As part of their willingness to partner, Habitat’s home buyers invest hundreds of hours of their own sweat equity working alongside volunteers and other Habitat homeowners.
2-Habitat Housing tends to be seen as a stabilizing force in the community. Habitat also does something called "A brush with Kindness" where they might work in the surrounding blocks improving the exterior of non Habitat houses as well.
3-Trends with Housing-The family tends to stay in the house for a long time. Typically the way that the financing works, a 1st and 2nd mortgage on on the property for a $100k House it might be an $80k 1st note and $20k 2nd. The 2nd note does not need to get paid back unless they go to sell the house in the first 10 years in which case it would be do. The goal is to provide affordable housing while eliminating the thought that someone would try to flip it.
4-If the community meets your criteria, I would consider having a Habitat house as a sign that stabilization or more investment is coming.
Overall Habitat is a great organization that is always in need of help and volunteers. There are all sorts of roles depending on what you're interested in.. Building, Operations, Finance, Family Selection, Fundraising, Marketing, etc