It is a nice machine. But...... you'll want to change the cpu card. And AGA lack of speed will annoy you and you'll see those empty Zorro slots that just HAVE to have a graphics card. But then you'll miss AGA games, and you hear about these things called "Indivision". And wouldn't it be great to have a burner...oh and a fast hard drive, but that needs a faster interface, like say a fastATA 4000....but yep its a nice machine...
Forget this....just enjoy your new (and very capable as-is) A4000. Your 040 A4000 can do 95% of anything that was ever required by an Amiga. Get the FULL use out of AGA and your 040 before you spend money on expensive and "previously owned" flakey expansions. When you're done that (several years of fun use) Jens may have come up with some new, reliable and affordable all-in-one A4000 expansion hardware if you still feel the need to upgrade.
I do get what stefcep2 is hinting at though....the desire to "upgrade" can be a vicious and expensive circle. Try to stay out of it and just enjoy what you have.