Neil Kinnock was going to be advised he would need to "spend a lot more on clothes" as prime minister if Labour had won the 1992 General Election.
Newly-released archive papers show the plans civil servants had drawn up for a possible transfer of power.
A memo outlined the pay and expenses Mr Kinnock was due but warned some costs were "not borne by the office".
Officials were also expecting Glenys Kinnock to play a more active political role than John Major's wife Norma had.
He was the Welsh politician tipped to become prime minister. Twenty-five years ago Neil Kinnock led Labour into a general election he was widely expected to win - only to lose narrowly to the incumbent Conservative John Major.
But newly-released official papers from the National Archives show how Downing Street was making detailed plans for the Islwyn MP's arrival in Number 10 - right down to the higher cost of living.
Neither Neil nor Glenys Kinnock would have seen the memo, which an expert has said would have been on the new prime minister's desk the day after the election.
It was written by Andrew Turnbull, then the prime minister's principal private secretary, and is headlined "Pay, Pension, Expenses, Tax".
He wrote: "I do not suppose the money has been the most powerful of the incentives propelling you to No 10 but for the record you might like to know what you are entitled to."
Mr Turnbull set out how much the prime minister would earn - £76,234 - and the "understanding" with the Treasury and the Inland Revenue under which some costs could be offset against tax.
"As prime minister, the costs of entertainment in your official capacity are set from the No 10 budget which also covers such things as gifts exchanged on official visits," he said.
"But you will find that there are a lot of costs which are not borne by the office but which you will incur simply by virtue of being prime minister.
"You will spend a lot more on clothes than if you were a private citizen. Mrs Kinnock likewise, especially for formal occasions."
Mark Dunton, contemporary records specialist at the National Archives, told the BBC about the memo: "It would have been there ready for the prime minister's desk on day one amongst a number of other briefing papers - quite an amazing amount.
"But even then they took a lot of trouble to steer the prime minister through so they say 'these are the decisions you need to consider in the first week, this is what you need to consider in the next week' and it's all laid out in a very helpful way.
"It does underline claims that have been made that the British civil service is the best in the world."
Mr Turnbull - now Lord Turnbull - had been liaising with Mr Kinnock's office about a possible transfer of power.
In another "secret and personal" memo released by the National Archives, he wrote: "The Kinnocks would live in the flat at Number 10 and intend to sell their house at Ealing.
"This probably implies greater use of Chequers [the prime minister's country home].
"I predict friction between them and the curator."
Mr Turnbull said Glenys Kinnock was "likely to play more of a role as spouse of the prime minister" and may need a press officer as well as the existing secretarial support given to Norma Major.
The Kinnocks never made it to Downing Street - or Chequers. He resigned as Labour leader within days of the election defeat, later quitting as an MP to become a European commissioner.
Lord Kinnock declined to comment on the archive revelations of what could have been.