Radial make great DI boxes, but Rupert Neve DI (RNDI) is my favourite for really clean capture. LR Baggs has a nice unit for acoustics. Samson makes a cheap but effective one. I would recommend an active DI for electric guitars.
Depends on whether you need to use speaker emulation (but if so remember to use an adequate load if it's a tube amp) or just intend to go direct to your interface.
Assuming your interface does have an 'instrument' input setting, it may be adequate. absolute difference between top DI's and competent ones isn't huge, but again, depends on your applications....