Can you negotiate a handover period? It seems a bit harsh to be dropped in it all at once.
As far as the book-keeping/tax/wages part goes, does the business currently employ/use the services of a book-keeper? If not, I would
recommend you do so at least for a few months while you are finding your feet. The sort of stuff is quite time consuming and you could spend a lot of time looking at it instead of managing the business. Much better to outsource it.
As far as the business goes, you need a good strategic oversight.
What are your fixed overheads (people, premises etc)? Where and when can you expect cash to come into the business from orders? You need at least a 12 month working cashflow. Watch the cash like a hawk at all times and remember most businesses fail due to cashflow problems.
Sounds daft, but really understand the difference between net and gross (I have known really smart people fail to grasp this). A £20k order does not look so good after you have deducted tax, a pro rata part of the costs of the employees who made it, a share of premises costs, materials etc.
Understand how many widgets you need to sell at what price in order to pay outgoings and break even. How many more before you turn a profit and can pay yourself, how many more before you can think of expanding. You must know what it costs you to fulfil an order and how profitable that order will be for you.
Many, many businesses fart about fulfilling orders which are simply not worth it for them financially. Being busy is not enough - busy AND profitable is the aim. Be ruthless, prune the clients who are difficult and the clients who are barely profitable. You want clients who pay on time and are profitable and preferably are regulars.
Then look forward, how many orders are coming down the pipeline? At some point, if you are not bringing in new business, you will come to a grinding halt. What is your plan for getting them in. Try and achieve a smooth timeline so you have a steady flow (Ironic laugh), what is your lead in time from first contact with a client to first order. Even if you are currently overrun with work a portion of your time must go into selling.
Six months down the line look at your staff. Are they the right people? do they all pull their weight? If it is a family firm I can more or less guarantee you will need to make some changes.