Yo bro, u sold yr 318? goin for new ride ??
IMHO,the question you post is kinda relative and can be difficult to answer as each individual has a very different profile. Being a 523 owner, let me share with you my personal experience.
For a start, without fail on a annual basis there is this rd tax and ins, i reckon at least $4 to 5k a year will be spend on that. Petrol will cost you $4k to $5k on avg based on national avg of 20km travel. Then there is maint charges which i simply based on pure maint and svc and that can cost $500 to $2000 a year.
Lastly, not forgetting the annual car dep and for a 5 series, the loss is especially huge in the first yr amounting from $40 to $50k and i personnally adopt a $20k to $25k loss on a per yr basis(i mean for a e60 523). So if you will to total up the cost involved. In a nutshell, the yearly cost will be at best $30k though i choose to believe $40k is a more feasible figure.
If you follow % as yr annual income rule, ask yrself how much you are willing to allocate to this car assuming it don generate any income for you.
Oh yes the above do not factor in any mod yah.
Having say so, let's us not be too sticky over facts and figure. cos in life, you only live once and if everyone will to standby such rule, taxi biz will be booming.
End of the day,as long you live on yr income. I suppose there is nothing wrong in rewarding yrself over a ride you hanker most