We can only talk here about various hosting plans offered by Lunarpages (and not other hosts). Are you looking to sell software directly via a site, or sell accounting services?
A year is not a long time for an established hosting company. Any host can give a discount for annual payments (over monthly), simply because it's a lot less costly to bill once a year, and hold on to customers for at least a year (setup and takedown costs). For a Lunarpages Basic (shared) hosting plan we're talking like $60 a year (annual billing), which is pocket change for a business. LP does not offer monthly billing plan, as it would be considerably more expensive than annual billing.
Go with an established host (such as LP) and don't worry about whether they'll be around a year from now. As for which LP plan to host on, unless you're transferring over a known customer base and usage amount, just start with Basic. If you outgrow it, you can always upgrade to a higher capacity plan (such as VPS/cloud) and transfer the unused balance to the new plan.