Your FileMaker 7 solution should work well on FileMaker 11 although I have seen some issues with layout objects that have transparency. Check your operating system to ensure that FMP 11 will run on the machine(s) that you have as there are older OS versions supported by FMP 7 that are not supported by 11 (and newer ones supported by 11 that are not supported by 7).
The only alternative to purchasing the needed licenses (1 for each computer per FileMaker's license agreement) is to do Instant Web Publishing (IWP) but you will undoubtedly need to make changes in your database to accomplish this because web browsers don't always have the same fonts/sizes/etc and there are some things that can't be done via IWP. FileMaker Pro 11 can host up to 5 simultaneous Web Publishing clients. If you need more than 5 IWP clients at one time you would have to step up to FileMaker Server 11 Advanced for about $3,000 which can handle 100.
Many thanks for all the info. I've never got on very well with Instant web Publishing, but I may have another look at it before I decide whether to make the upgrade.