Large FL Studio projects benefit from a decent CPU (i5/i7) and unless you want to use a multi-screen setup, the iGPU should be sufficient.
A SSD drive is always advisable for overall system performance and 16GB RAM would be good.
For music production you should get a ASIO/ASIO2 audio-interface. I don't know your exact needs and am also not well versed with them.
In the end It really depends on your budget and how complex your projects are.
I don't know if there are good price-performance All-in-One dekstops, because I never bothered with them.
That would be a good build you could consider if your budget allows it:
Intel Core i7-6700K CPU ~$340
Cooler Master Hyper 212 EVO CPU Cooler ~$30
Asus Z170-A Motherboard ~$150
2x8GB G.Skill TridentZ DDR4-3200 RAM ~$90
250GB Samsung 850 EVO SSD ~ $100
2TB WD Green Hard Drive ~$95
550W EVGA SuperNOVA G2 PSU ~$85
Corsair Carbide Series 200R Case ~$60
Microsoft Windows 10 Home 64-bit ~$100
ESI [email protected]
XTe PCIe Audio Interface ~$240
I know, that's far away from cheap, but it's future-proof for several years.
As I said, for the audio interface you should know what your needs are.
You could also read this buyers guide:
I hope the input helps!