The delays are often exagerated but every CAA should check for outstanding finance before allowing a change of ownership. This can take a while if a finance company has to issue a clearance. Many countries have exchange control legislation and will want to ensure that the seller is not taking money out of the country illegally. Avionics are not usually a major issue, an aircraft from South Africa (for instance) will come with an ADF which is a bit useless in the UK but you will have to budget for an 8.33 radio.
VAT is an issue - budget for 20% of the UK landed cost.
There are specialists who will advise you and do the job and it is worth using them because they know all of the little twists. For example it is often cheaper to take the wings off and pop it into a container than to pay for a ferry flight.