Depends how much it cost and how much your willing put into it. You can get a used m54 engine for quite cheap with lowish millage.

m54 is a great engine ive got the 3.0l in my z4 and it runs like a dream can easily get over 300k with good care. however you dont know the exact care from past 21 years and 267k miles.

You’ll also have all the problems that comes witj m54 engines gasket seals etc even tho im pretty sure 2.5l didnt have as many problems

Id say if body is good and price is cheap and you have money to put into it go for it!

Some idea of expenses (obviously this is personal to me) this is is what Ive spent on my bmw z4 since owning.

Replacing interior trim (dashboard, radio, vents, cigarette console) ~£200 second hand

Complete interior overhaul (Refoaming leather, dying whole interior, fixing couple of cuts in door frames, leather upholstery) ~ £600

Roof motor replaced and repositioned: ~£200 (personal to convertible)

Spark plugs: ~£40 each

oil pump nut upgrade: ~£300

oil leak and head gasket ~ £400 + ~£200

for oil leak and head gasket I cant remember exactly what was done as that was one of first things I did apologies but this was all within a year to get things closer to how I want. Its a reliable engine just the age of it buying it second hand where you can’t trust its history can be pricey.