Almost all of mine were on soy formula, I know your concerns, my know 15 year old DD is vegetarian, and we sub with soy "meats". Of 7 kids only 1 could handle milk based formula, and all of them are fine, your baby will only be on it a short time of his life, all the research looks at it over a long period of time, So I really wouldn't worry about it to much, Even know I substitue w/ soy but with everything in life it must be in moderation, especially over long periods.
Know to the constipation my children had this problem even on soy based, what my DR had me do is add an extra once of water to the formula, and then give a little watered down apple or prune juice, about once ounce water to one ounce juice, once they were regular we went to every other day. I always used or substitued (when I ran out of WIC formula which I didn't always qualify for) the Wal-Mart brand formula, formula is regulated and they all have to meet govermant standards, just like prescriptions. Read the labels and compare you can see for yourself, and it SO much cheaper, at least until he is ready for milk, I would wait until at least 11 months before introducing it to him. I learned from my Dr with #6 who was born with only one kidney that it increases there chances of allergies later in life and that it can lead to, I believe it is kidney stones, his reasons were that little ones are not set up to hold the amount of calcium that is found in regular milk, this was from her kidney specialist.
I hope I helped a little, good luck, constipation is no fun!