Obviously not a doctor but have done a bit of reading for myself
Might be worth having your levels checked before supplementing in case you are deficient. You can test Vitamin D whilst supplementing but have to be off Vitamin B12 for four months to get a true reading.
Vitamin D - gel capsules of D3 if possible. Recommended dose varies according to maintaining or increasing your level. My capsules come from Costco and are 2000 iu - I take 4000 iu as I've been Vitamin D deficient and need to keep my level up.
Magnesium comes in a variety of forms some better absorbed than others eg Magnesium Glycinate or Magnesium Malate. I take 400 mg but up to 800-900 mg ok, if tolerated.
Vitamin K2 (Mk4 or Mk7) - important to take with Vitamin D if dosing at higher levels. It keeps calcium in the teeth and bones rather than potentially circulating in the blood. I take 5mg of Mk4.
Vitamin B12 - NHS deficiency levels (below 200) a lot lower than some other countries eg China says you're deficient if your reading is 500 or below. I used a sublingual spray 1200 mcg daily.
Biotin - good for hair and nails. Helps control my hair shedding - I take a high dose of 10 mg.
Lots of information on the internet, worth spending some time having a read. Not sure if I'm allowed to recommend websites
Hopefully others will come a long with suggestions then you can decide what's right for you - good luck.