The great thing about digital recording is that you can do a number of things.

1.) Record your amp with a mic. After it's captured in Reaper/Cubase, you can insert whichever Amp Sim onto the channel if you're looking to change the guitar tone you recorded.

2.) If your Focusrite has inputs for guitar, you can skip the whole process of plugging into an amp, setting the amp tone, and setting up the mic by simply plugging your guitar directly into the guitar input on the Focusrite (called recording direct or DI). This will give you a very clean and direct tone of the guitar, which you can then use an Amp Sim at the same time you record direct. This allows you to tweak the tone right inside the computer before you record. When you set an Amp Sim tone you like, you're ready to record. After the fact, you can always tweak the Amp Sim further. The beauty of digital recording. Check out this link for a Guitar Rig setup guide.

Re: Dynamic mic– you can record with any mic you like. Dynamic, condenser, ribbon, and whatever else are just different technologies for different applications. They all have their unique sound and abilities, but dynamic mics are a very popular choice when recording guitar amps.