In the England & Wales an executor can be a beneficiary.
You cannot be a beneficiary (or the spouse of a beneficiary) and a witness to the Will. This would invalidate the gift, but not the Will
In Scotland its the same, but I'd go and check with your solicitor, you'll need one for the IHT200 anyway and then there's the C1, sends a cold shiver down my spine just thinking about it
You do NOT need a solicitor. If the estate is straightforward and you are reasonably intelegent you can do it all yourself and save a lot of time and money.