Works traditionally: long, with mass of useless activities and time spending.....:

What realtors generally do:

Just finding real estate facilities under requirements.
Just provide legal assistance.
Not all real estate brokerage providers differentiate buyers and investors.

Extended investment / purchasing model description:

Get the requirements to expected rental profitability and a budget of investment,
To inform about current variants, their rental profitability and a quality of tenants, the % of vacancies and last trends in vacant spaces,
To calculate possible variants with extended purchasing model: rental profitability, valuation growth of an asset, ROI in 5 years investment period,
To offer the best 3-4 variants,
To choose an optimal variant to purchase,
Buy the land plot,
Prepare a draft of the purchasing contract,
Make a complete assistance of a deal,
Define a payment method, reliable for both sides of a deal,
Organize a transfer of a property with signing a transferring document,

NEXT:
Choose the best prefabricated buildings deliver / contractor with the best prices,
If it is necessary - organize a tender for selecting of deliver / contractor,
Organize the process of obtaining of construction permit,
Choose a contractor with the best price (if a prefabricated buildings manufacturer is not a contractor for building installation),
To organize signing contracts with utilities providers,
To supervise for construction process,
To hire a qualified engineer for technical supervision of construction process,
To get a permit on commercial exploitation of the facility,
To manage of facility on behalf and for the interest of the owner,
To hire a qualified engineer for technical maintenance of the facility,
To attract long-term tenants for the best terms,
To manage lease relations ,
To provide bookkeeping and legal assistance connected to the property - to hire qualified lawyer and bookkeeper for this purpose,

Next:
To send regular reports to the owner about the technical state of the property.