You cannot as such change a Limited Company to a sole trader. You can wind the company up and "sell" the trade and its assets (and liabilities such as they are) to a new sole trader business. It is in all legal respects a new business though.
You would almost certainly be better advised to retain the company though as Salas suggested in order to preserve the name. That would cost �30 per year at present rates and you'd need to file what's known as dormant annual accounts at Companies House. You don't need to maintain a bank account in the company name though.
It's perfectly legal for a company to operate at a loss, though it isn't legal for one to operate if it's insolvent which is a different thing altogether. Insolvency means the liabilities are greater than the assets. You can make a normal loss and still have sufficient assets from past years to be operating legally.
The position with regard to debts of the company will depend on the terms of those debts. They probably won't leave with the previous directors though. You need to be very careful in such circumstances and take professional advice. That is why most business purchasers merely buy the assets of the business they are taking over and form their own new company to run it, not buy the existing company. Your solicitor is correct about that.