Each suburb of Mumbai offers you a unique aspect. Some are very close to the business district. Some are a little faraway but provide you comparatively decongested spaces. Some like panvel and navi Mumbai provide you with well planned infrastructure. Navi Mumbai(meaning new Mumbai) has well planned infrastructure and plenty of commercial opportunities as well.
Navi Mumbai also is an viable option for investing in property in Mumbai. Navi Mumbai is well connected by train and road to the business district of Mumbai. Also a new international airport is coming up in navi Mumbai, which will make it a very well sufficient part of Mumbai in terms of business as well as residential opportunities. Also many corporate giants have there headquarters in navi Mumbai.
If you want to buy property in Mumbai at a place which is very well connected to the entire Mumbai then you can go with thane. There are many flats available at comparatively cheaper rates(cheaper then in proper Mumbai) in areas like gb road in thane. Also thane is very well connected to navi Mumbai as well as Mumbai by train and road both. If you want affordable property as compared to the property in Mumbai and are willing to move away from Mumbai then you can try suburbs like Dombivlvi and kalyan.
If you are looking to move further away from Kalyan then Badlapur and Ambarnath are much more affordable and have a great environment with less pollution and cleaner air. These are well connected to Mumbai by train and road(by train 80-85 mins away from dadar and 35 mins away from thane).
Thus these are some of the most viable options for buying property in Mumbai.