Not sure who would be right for thorough advice, perhaps an independent financial advisor who has experience with Shared Ownership.
Freehold means that you own the land and building of the house. It's in contrast to Leasehold. Leasehold means you lease the right to the land for the duration of the lease. Whilst a SO property is below 100% they are leasehold. Depending on the contract, depends on if you will be given the freehold at 100%. You want to be given the freehold. If you're not, then you may have to pay extra to buy it, or be unable to obtain it.
If you're unable to obtain it then you may struggle to sell if the lease is below 80 years, be subject to ground rent/service charge, and have to request permission to make structural alterations.
Being leasehold in the first place isn't a huge red flag for shared ownership because they all are. But the red flag is there if you're not automatically given the freehold at 100%.
If you're going to buy 25% with a 3k deposit, and a 32k mortgage. How do you intend to buy the other 75% later?
When you're working out if it is better than renting, don't just factor in the monthly costs. You also need to factor in costs like mortgage product fees, advisor fees, solicitor fees etc. If you plan to Staircase (buy more shares) you have to pay for a valuation, your legal fees and the fees of the housing association. So it can cost a few 1000 each time. So you don't want to do it in small chucks (might be different under the proposed new model that will come out in the near future).
So if you could buy a house without SO in the next 5 years, it may be better to wait.
Please also be aware that the rents increase. Some contracts will put it as high as 5% per year, so if you don't have much room in your income to accommodate this, it could become an issue.