Setting up bulk mailing server for few clients for their email newsletter has been a long time goal for me.

Something on the lines of mailchimp.

PowerMTA from port 25 and Greenarrow seem to provide with full fledge web software which simply runs in linux environment. Both handle bounces, multiple clients etc.

The question is, should I go with them and purchase ridiculously expensive stuff they are offering, or configuring postfix in right way would enable linode to send around 100,000 email in an hour ?

Sure you can do it yourself if you want, but you'll have to deal with those issues yourself. On the other hand you could use something like Amazon Simple Email Service if you are prepared to slowly ramp up your requirements rather than just immediately dumping 100,000 emails on their service.