With only 10-15 users and the system working correctly, I would not upgrade to SQL Server. If the system is working slowly because you have too many records for Access to handle effectively, then I would upgrade. The world being imperfect, however, means you'll probably have to do the upgrade anyway.
First to answer your questions. SQL Server has no front end, you'll have to create one or use the Access one you've already built (with tweaking of course). Not sure if you can link SQL tables in Access 97, but I know you can in 2000. Go ahead and experiment to see if you can link a SQL table before you decide on your upgrade strategy.
Speed issues will not be a problem as SQL is considerably faster than Access. That said, you may need to rewrite your queries into SQL stored procedures to gain the benefits of this speed.
Of bigger concern is the fact that SQL Server requires database administration whereas Access generally does very little of this. You'll need to create an effective backup strategy. In SQL backups sereve two purposes. The main one of course is to be able to revoer the database to a particular ppoint in time. But something people converting to Access often don't understand is that it also clears the transaction log. So they continue to rely on the network backup as they did with Access and then a few months later they are on this board asking how come their transaction log is now 15 gig and how to clear it out. You'll also need to learn about setting up user security if you weren't using it in Access (as many people did not). Another issue you will have to address is performance monitoring and tuning. Another issue for you is that Access SQL and T-SQL (SQL Server' version of SQL) and not the same and you will have to learn some things over again. At least you wrote SQL statements, many people converting from Access only used query designer and never even looked at the SQL statements before their conversion.
Good luck. Keep coming here for help as you move through this process and get yourself a couple of big SQL Server books to use as guides (You'll also have SQL Books on Line, but it is often less than helpful to the beginner user). Hey, the comapany is forcing this change, see if you can get them to send you to a training class or at least buy you some of the books. In your frustration during this change, just remember you are gaining a professional skill that is worth more on the job market than Access programmers can command. Go to
to see what dba's in your local area make. You should be able to use this information to get a pay raise! In my area, dba's make about 30K more than I made as an Access programmer. (I didn't get this much when I converted, mind you, but I did just swing a 17% pay raise based on this information.)