Are you sure you'll be too late for a train? - the last one's generally about 11pm-midnight. Timetables here: http://www.sbb.ch/index_e.htm
You can buy a rail pass (various types available) from the Switzerland Travel Centre before you go which will give you a saving on the train fare. The cheapest pass is the Transfer Ticket, about £45, which gives you one free return journey within Switzerland. Details here: http://uk.myswitzerland.com/en/welcome_frame.cfm
The postbuses and trains are part of an integrated transport system, so the routes don't overlap much. You would probably have to change 4 or 5 times if you went by bus, so it's not really viable.
The route you mention is also known as the Italian High Level Route, the Italian Haute Route or the Monte Rosa Traverse. It's more usually done in the reverse of the direction you mention, ending on Monte Rosa. It is very popular, so you should be able to find plenty of info if you search.
Book huts in advance if you're going in high season. I assume you have some Alpine experience, as this is not a complete beginner's route. However, it's relatively straightforward (max AD-ish), and you can avoid the most technical part, the Liskamm ridge, if you wish by traversing below the south face instead. Altitude effects really sock in at around 3500m, so acclimatise for a couple of days lower down first to make the whole experience more enjoyable. If you travel in the 'normal' direction, the highest point comes at the end, which will also help.
