The pipes will be a constant maintenance expenditure, and the main is a ticking time bomb that will cost tens of thousands of dollars to replace and require emptying the entire building, and taking out the floor of several bottom floor apartments.The math nerd in me wants to quantify the risk - what's the likelihood of the main going while I own it (I only plan to own 2-3 yrs), how many replaced pipes per year average at what cost per break, etc.

There is a ton of great information on this site about estimating expenses, but here are a few items you'll probably want to account for in addition to the mortgage/financing costs:Property taxesInsuranceVacancy factorRepairsCapital expenditures (major repairs)Property management

Are you adding capital expenditures in to your expenses?

Though I wouldn't count on appreciation in determining whether or not to buy, you probably have a pretty good chance of having it in the future.The bad: You actually will not have $400/month in cash flow after you account for vacancies (generally 7-12% of rents), mx reserve (generally 10% of rents), and capital expenditures (generally 5% to cover large items like roofs/foundation/sewer/etc.). 

Utilities, trash, water, vacancy, maintenance, capital expenditures (roof, furnace, etc), etc.

Jettison a property at the peak of profitability before having to sink significant capital expenditures into the property.

That's to cover capital expenditures that might come up over the years - a roof here, some HVAC there, etc.

As far as determining a reasonable asking price, you will need to see the NOI on the property, and determine the CAP rate that is the going rate for your area, then factor in any capital expenditures that you anticipate encountering such as replacing the roofing, HVAC, etc...After that, you'll want to determine what the cashflow will be and make sure you'll be able to service the debt, set aside money for vacancy, repair, and property management, and however much you'd like to net in cashflow.

From my observations, building credit is easiest and safest with predictable and regular expenditures (like gas and groceries).

I am also curious how people look at capital expenditures and whether or not that budget is part of paying for maintenance replacement items, or just for big ticket items like roofs and furnaces.