Technically possible? Definitely.
Complicated? That really depends on the details: How many sites need to be scraped? How much information is being gathered? How will you aggregate data from all these different sites? You'll want to complete a detailed analysis of your project before writing a single line of code.
Legal? Probably not... you'll have to look up the terms and conditions of every site you scrape to see if they're cool with you lifting their data. If in doubt, send an email to the website owner and ask. If your site will be competing with them, obviously they will have a problem with you using their data, and they may take legal action to shut you down-- especially if you're successful/popular. This is not something you want to ignore; if you don't obtain written permission up-front, you may go through a lot of work only to be shut down overnight. And believe me when I say, hosting companies do not mess around with sites they're hosting that may be in a legal grey area-- they will shut you down first and ask questions later.
If you get a green light to scrape, then you should put together a business plan, assuming you intend to make money on this website. A good business plan will help you determine where the money will come from, where your visitors (customers) will come from, how you will market the site, who you will market the site to, how soon you will expect to be profitable, etc. etc. etc.