Is 150 CFM ventilation fan right choice for bathroom in my older condo?
Calculate the CFM Needed.   (Length X Width X Height)/7.5  (7 x 5 x 8)/7.5 = 37.3 CFM   Did the 7x5 include a Shower/tub area too? If so, the 50 CFM should be fine. 

That Fan is a bit overkill for your bathroom, that said Mine is a 190CFM for the same size.  The issue you are going to run into is that the fan you chose has a 6" Duct coming from it. 

Typically fans from a Bathroom are 3" or 4" for something that size. It would be Similar to your Dryer Vent where it Exits the house, so that should give you an Idea.   If you can get the Part number of the Existing Fan and look it up you should be able to google it to determine its Duct Size.   There are Downsize collars that would downsize the 6" to 3 or 4" though that is not recommended as you will cause back pressure on the Fan as it can't push the required CFM out the smaller hole. If your Duct run was less than 2' you might get away with it, but if not you would probably void the fan warranty. 

Find the Duct you have and then get a Fan that Supports its.