I rarely do private sales and these tend to be older low value cars. WBAC on a 2009 TT £4000, sold for £5800 within 24 hours, new purchaser happy as Larry with usual disclaimers. Not sure where 9 months and £700 a month comes from in your example. I bought for 60, Dealer sold for £57500 and I pocketed £54.7k and that was over 18 month! Current car from Sept 69 plate so 16 month and figures look very similar. Dealers asking £57k, I should return £54-55k if I sell so.. 60 -55 divided by either 18 months ( 18-69) or 16 months currently so to be crystal clear this is no where near £700 a month.
As for pensions, each to their own on that one but mine aren't increasing by 7% in the current Pandemic Brexit bolloxcks environment.
I took the £55k as the trade price of your current one, knocked off a couple of grand for the dealer margin, and divided by 9 if it was a late 69 plate - oh and took just over 60k as 61k in true man maths style. Son in law works for a large dealer group so we are fully aware of margins and prices and where the profit is, and it is possible to run a new car cheaply if you buy well... but equally you can get stung. For example Porsche owners witter on about residuals on their cars and yet they are normally looking at dealer sale prices when the average margin on a secondhand porsche is £7k....
You need to do it yourself and remember its all about time in market, as I dont intend to retire this year, returns this year are irrelevant. However, dropping in a lump sum whilst prices were ;low back in April might have been a good idea ....