Do you plan on milling for customers or just selling cut lumber to customers?
Ideally you will want a csm and bsm, both have there pluses and minuses. As you have a tree service and equipment to handle logs your laughing.
Cost wise, you will find that a bsm is more economical although requires a greater capital investment, I would suggest a manual mill such as an LT10, ML26, etc types.
Capital outlay would be approx less than 5K USD to be set up well. Production is greater with a bsm, faster, easier, less fuel costs etc (my csm can burn an entire tank in one 10 foot cut, my bsm runs all day on two quarts and can push close to 1000bf/day. Even with a BSM you will still need a csm, for unique logs, getting others down to size etc.......
Another thing, if you own a csm or bsm and get bitten it can cost alot if you are married, divorce lawyers can be costly!