Modelers sound good. Many pro recording engineers will roll their eyes if you come in without one. Line6 and Roland make respected units. If you don't want to model, you'll be mic'ing your cabinet or combo.
A modeler is a preamp. A preamp amplifies a very weak signal, say directly from a microphone or guitar, into a line-level signal accepted by a mixer, recording device, or power amp. A preamp is not necessarily a modeler.
You need:
audio source -> preamp -> audio interface -> computer w/recording software
So:
guitar -> preamp/modeler -> firewire/usb/pci card -> pc/software
or
guitar amp -> microphone -> preamp ->firewire/usb/pci card -> pc/software
And:
vocals -> microphone -> preamp ->firewire/usb/pci card -> pc/software
A lot of audio interfaces will have preamps on board for the mic inputs
so you could do:
source -> microphone -> firewire/usb/pci card w/preamp -> pc/software .
So with a mic or two, a nice audio interface, and a computer with decent recording software you can potentially record your guitar and vocals to disk.