Sounds like you are keen on E39, look for tidy 540i?
In a word... Budget... A nice, late low mileage 540I Touring is £6-8k... Even an earlier saloon needing work is well over a grand
For £3-500 ish, you can buy: a 525i needing work, an Omega needing work or anything else needing work...
Generally the E39 will need a rad and breathers/oil seals... The Omega will need a cambelt, wishbones, Camcovers, geometry and most likely, tyres, plus everything else that the last four owners have neglected...
Bearing in mind that several of the Omegas on ebay have been for sale for over a year at £7-895, a similar age/condition E39 will sell for twice that...
The conundrum is simply that the Omega is a familiar beast that I have a soft spot for, whereas the E39 is much nicer to drive... Day to day running costs are identical... Same economy, same tyres, same insurance, parts are a touch cheaper for the BM too