The instrumentation dictates it'll really have to be multitrack. You could do vocals and acoustic guitar at the same time, but leakage from a drum kit will be too difficult to control with that at the same time.
You say it only needs to be a crude recording. A really cheap way to get a decent result is using some stereo electret mics. You can buy these from a famous auction site, try the search term "electret mic". You'll see many STEREO MINIDISC / LAPTOP ELECTRET MICROPHONE MIC items. These are very respectable quality mics and will produce great results with the acoustic guitar and vocals with minimum budget (£17 or so).
They are designed to work with minidisc recorders however, as they draw power from them via a special system. They say they also work with laptops (and presumably desktops mic sockets?); those sockets are mono though, so I don't know if you'd get stereo
I haven't tried them myself, but use identical capsules in my own mics (which are phantom powered), they are excellent for the money, much better than anything else at that price level, especially for your instrumentation. The only issue may be the level of the drum kit (assuming its a kit, not percussion?). The mics can be overloaded so must be placed some distance from the kit to prevent distortion
I don't know if you mabye wanted a higher quality setup, but this can produce a decent result very cheaply. If you do, specify a budget
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