I'm thinking there would be few poorer investments than purchasing an existing roofing company. Why? It is a business that doesn't refresh itself except for every 15 to 25 years. Do a roof today, unless you're in a hail zone, you're not likely to see that roof replaced for a long time. However, when you purchase that company, the installed base of Customers and warranty obligations come along with the purchase.
Okay, admittedly, there is something to say for having an established A rating with the local BBB, perhaps a great Angie's List record and reputation and you're "established" and "proven". But what is that really worth? If it was a good, relatively large market and the company being purchased was one of the 2 or 3 most recognized name brands in that market, I might consider paying $50K for that alone. However, once again, if it is one of those 2 or 3, then it would mean their volume of work per year is relatively high. You could be inheriting warranty obligations for 500 to 5,000 roofs (or more).
You spoke about leads. IMHO, if you're thinking you're going to have a successful company by sitting back and calling on leads, save yourself some money and continue working for someone else. You create leads yourself. You do that by canvassing/door knocking, social networking, door hangers, advertising, etc.. Unless you're in a really small market, perhaps a rural area, I'm thinking there's way too many competitors out there hustling business for anyone to expect their business can be sustained and grown by sitting around waiting for leads.
I'm not really into partners myself. Good way to ruin a friendship and go bankrupt I think. If I'm reading it right, I'm guessing you're looking for a partner that would front most of the money and then you'd do your part with sweat equity. Turn that around for a minute and image you're the guy with $250K to $500K to invest in a business. Are you going to dump that in and give another guy you may barely know a 50/50 deal to just "run it" for you? Let's say that guy already failed a business when he ran out of leads in 3 weeks.
I'm not trying to be harsh or negative, you asked the question, I'm doing my best to give you a realistic answer. Save a fair amount of money and if you're still inclined, start your own business from scratch. Do that after you've prepared a detailed business plan including sales and financial projections. Share that with at least successful, experienced business people you know and ask for the feedback. If the feedback is positive, then consider doing it. Just to help a bit, the most important thing of all is financial. The vast majority of businesses fail in the first 2 years because of cash flow. Do a personal budget and determine how much money you need for you and your family to live a year. Okay, you need that amount in your checking/savings account. You probably need $25K to $100K, depending upon what type of roofing business you're going to open, on top of that. Perhaps more. If you understand the insurance side of things and are in a decent market for that, you could do it on the low side if you sub everything out.
Good luck to you, I hope you're successful in your endeavors.