I'll take a stab at it to get things started.
Are you prepared to be available to rent out units when the customer is there to fill it?
You might want to find the applicable laws regarding how you get possession of a unit
when the customer doesn't pay, and how many months rent you lose while getting possession.
And then how you must dispose of those possessions.
How much maintenance is required immediately, and how much each year.
Da money!
First, to make it easy I'll do cash on cash, forget any leverage.
Can you get another $1,000 for the 3,000 ft shop?
I have to assume something, so...
$12,000 for the 30 units and $12,000 for the shop. Gross $24,000
Expenses, Taxes, $1,400, electric, $1,200, maintenance, $2,400, insurance, $1,200 and , $1,000 as a pad.
Total $7,200, then $24,000 - $7,200 = $16,800.
$16,800/$135,000 = 12.4% return on cash.
However, If you spend 10 hours a week 50 weeks a year, that 500 hours.
Giving yourself $12hr, is $6,000. Adding that to your costs will bring the return down to,
$10,800/$135,000=8%
You can go ahead and rerun the numbers with 20% down and a mortgage payment,
although I don't know if a bank will lend on such a property with 20% down.
I'm sure someone more experienced will add some things I missed and may be a little less conservative.