For now, we want to get the website up and running - just with a sign-up page,
Right now, just set up a static web page on a 10 USD/month shared webhosting somewhere. Or use a service like Performable (they seem to be moving away from landing pages ?). And register for Google Webmaster tools, and start getting your website indexed. Then switch to your 'real' hosting platform when you're ready and cancel the shared hosting account.
and a simple blog
Self-hosted Wordpress takes some work. Hosted Wordpress is easy to get started with, but locks you in on the URL structure "blog.company.com" (because you need the separate hostname "blog" to point to the other provider). Think about SEO, how much energy you'll really invest in blogging, and your in-house sysadmin skills, and make a decision and move on.
Amazon EC2 is a good choice for a LAMP B2B SaaS
Sure it is. As would Linode, Rackspace dedicated servers, Rackspace Cloud, Gigenet servers, and a number of other hosting platforms be. Small- to mid-scale PHP hosting is ubiquitous.
What you should mainly think about is where you'll choose to split between in-house sysadmin responsibility (securing servers, patching servers, database administration, data backup), and external fully managed services.
(Nota bene: Amazon has completely different maintenance levels built into their different products. Their hosted load balancer and hosted MySQL are fully managed, but EC2 servers aren't managed by them at all (you must do backups, OS security patching, etc).)