To my knowledge, you can take just B12 without extra issues, though it probably won't do as much for you as a targeted set of supplements together. You can search here for the "Simplified Methylation Protocol" or "Active B12 Protocol" for what others have found helpful in this regard. There are also different varieties of B12 (methyl, hydroxy, adenosyl, cyano) and there is a bit of debate here over which are most helpful or best to take (though there is relative agreement that too much cyano isn't a great thing). I would say that hydroxy is a safe choice, but that methyl might do a bit more for you. But that's just my perception. I take some of each.
If you're not doing injections, I would strongly recommend a sublingual B12 (i.e. dissolves in your mouth) rather than one you just swallow, as the absorption is generally MUCH better (let it sit in your mouth as long as possible rather than chewing it). You may also need much, much more than the "typical" dose in order for it to be as useful as it can be (there are reasons for this, but I won't go into them here). I use the brand AOR. They make methyl and hydroxy sublinguals, and are a professional high quality brand. I have also heard people here say good things about Jarrow's Methyl B12. With any supplements, you are probably best starting with a very low dose to see how you tolerate it before increasing. If B12 jump starts some of your body systems, you might feel crappy for a while. It also might be wise to make sure you're having some potassium rich foods in case your body needs the extra for making new blood cells. If you start to have muscle cramps, it is probably worth considering low potassium as the culprit.
Many people with ME/CFS seem to have issues using folic acid, and need a special form of folate (methylfolate - you can find it in supplements under the trademarked name "Metafolin"), and there is a little debate here on the folic acid issue. It's helpful for some, not good at all for a few others. Some form of folate is generally important for us to get, though.
A B complex is probably a good thing to take, but you are probably going to need much more B12 than what is found in one, and I would say that is definitely the priority.
Hope that helps at all.