@Linna Li
Congrats.
What work do you want to do? What needs to get done? How are you paying (cash vs loan)?
I have paid for a few renovations - and costs can vary a lot between contractors, so I would work on simplifying the scope (built in anything vs off the shelf is going to add cost).
I spent around $125 psf on a decent 3000 SF renovation (interior mostly) including carrying costs for the year it took. https://www.zillow.com/homedetails/44-High-St-Newton-MA-0246...
And - systems are important. Like roof, plumbing, heating, and electric. If the home is 1900 and plumbing has never been touched, and electrical is old, you might want to replace vs slapping new tile over old plumbing lines. Also, better to replace tubs vs refinishing.
If your going exterior, interior etc that may be $150 psf+. True guts of 3 unit plus buildings may require sprinklers.
We have done skin deep renovations for much less (updating kitchen cabinets, appliances, bathrooms) without truly touching systems (besides any required updates such as electric to each appliance).
And - if you are renovating - determine best rental layouts - might not be the same as condo layouts.
Happy to connect you with contractors and architects I have used.
And permit processes can be a pain - but important to have.