Look at a few ratios, such as return on equity, net profit etc ask for latest audited balance sheets, P & L and get a professional to verify it if you don't have the skilles.
That's also what contracts is for. In the contract, you will include the terms of the sale (i.e you can list what's the client, or even just attach the latest audited statements as what's being represented to you and what you are buying). You would also want to look into their assets, management team capability etc. Without more details, a pricing on the shares is not possible.
I would be interested in such shares if they go at a reasonable rate, and the company intends to pay out most of the profit in dividends (else the only gain possible is on liquidation of shares which isn't exactly very desirable).
It's being 25%, I would worry about say in management decisions (if that's your thing). Because, you can be outvoted on everything if the other 75% are in the same camp including in EGMs.