I assume all new macs will have Lion installed (when it's released).
Will there be an option to have Snow Leopard instead?
I need to be sure all my Applications will have zero problems on Lion before I commit.
Not once they start shipping with Lion. You'll have to install SL over Lion which may require blanking the hard drive. If there are hardware updates concurrent with the Lion release you won't be able to downgrade if you buy the new hardware.
Like before with new Mac OS releases I suspect once Lion is released there will be no option to buy a Mac with Snow Leopard unless you can find a third party retailer who still had old stock. Any new models will come with Lion pre-installed.
Apple has never done anything like that before, so probably not this time. If you really need snow leopard just buy now, and get your free upgrade to lion that you can instal when it is stable enough for you. Also as any new hardware updates will come with lion there will never be newer hardware that runs snow leopard.