You are entering what I believe is one of the single toughest sectors. Not only do you have other travel websites you also have tour operators, comparison sites, airlines, review sites and so on. Anything to do with this market is going to be immensely tough for a new site.
This is one of those areas where you need to get everything perfect and you need to be prepared to put in some amazing hours and many months to get the site out there. Be warned though Most of the big boys have spent tens of thousands and even hundreds of thousands to get their site at the top. It has taken them many years so competing against that is going to be very, very hard!
- Be prepared to wait for sandbox to end. You site will probably be sandboxed (be nowhere to be seen) for the first few months. This is normal.
- Content is king Your content must be amazing for both user and Search engine.
- Content is King You need a lot of it. Check competitors and see how many pages they have (google site:www.example.com). The more "excellent" content you have the better!
- Content is King People will want to bookmark it or pass it onto friends (digg etc)
- Think like a searcher. Would you be typing in "rome Italy hotel" or would you be more likely to enter "hotels in rome"? The correct title tag and content will break or break you.
- Incoming links You will need a ton of them...and then some...and then some more! You should devote a lot of your time obtaining links from other travel related sites to link to yours. The more you have the more chance of being high up you have.
- Directories Submit your site to as many directory sites as you can
- Content is King - Think of anything that can make your site unique. Could be reviews, forum, currency details, flight info or car hire. Anything that can add weight and make you stand out you need to do.
I know I sound downbeat but I have created content for this area in the past and it was incredibly tough to first page. I only managed it after many, many months of hard work. You cant even PPC without having to spend thousands (We blew a 1k in an afternoon just seeing what it took to get to the top right position).
Personally if I were you Id create two sites. One generic going after the big boys and another one which is very niche so at least you have two chances.