New owner wants to buy in for 500k which is above the approx value of the shares. Assumption of approx value based on current equity as no formal valuation has been done.

1) Does a valuation need to be done-required

2) Should the owners sell a portion of their stock 30% (15%/15% to new owner or should the Company Buy back the 30% and re-issue. What's best for tax.

Thank you in advance.