It's been about three years since I've had the On3 BMW N54 single turbo kit installed on my E90 335i. Through those years, the price of N54 335i's has tanked quite a lot. So looking at a cost-effective kit such as the On 3 N54 single turbo kit looks a bit tempting. Since most kits cost more than you paid for the car. I want to share my experience owning this kit for an extended time.
How are the turbo and wastegate?
The turbo and the wastegate are the parts most people are concerned about. I ran the GT35 61mm turbo with my setup, and it spooled well, and I couldn't be any happier with it. But after about ~20k miles, it developed shaft play. Enough shaft play that I can see where the blade made contact with the housing at one point. Once I noticed it, I promptly removed the turbo.
The wastegate held up fine and didn't have any issues with it. No boost creep or any problems with building or maintaining boost. So its not much of a worry, but even if you go with the On3 wastegate, or a well known brand such as Tial. Please take it apart and inspect it. Things like making sure the diaphragm isn't damaged or pinched, the piston looks well lubed, and even hook it up to an air compressor to see if it holds boost.
What would I change?
If I was to get the kit all over again, which I am. These are the changes I would make. The first one would be to buy the hot side parts, which will cut costs for you. Buying the hot side parts will only come with the turbo manifold, turbo downpipe, and wastegate dump pipe. You will still need to source other pieces to complete your setup, which some people don't want to do. If that's the case, order the complete kit minus every other option provided.
But since I went hot side parts only. I'd then order the coolant/oil block off, oil feed adaptor, and oil return pieces separately. Quite a few vendors offer these pieces. I'd recommend Vargas Turbos (VTT) to source those parts. Since the -10 AN front oil drain adaptor they provide allows you to use AN lines for your drain. This is important since you can run some quality hoses and ensure no leaks are present.
I've gone through two oil drain lines. Since worm clamps are used, they tend to rip hose, or after a while, the middle of the line will sag and leak. This is even with a heat shield over the line. I don't believe any kit that is released currently provides anything but the rubber lines. So save yourself the headache or a possible fire and upgrade those parts.
I'd also run AN lines for the wastegate and MAC solenoid lines. You might as well do it right and don't have to worry about those lines. I didn't have any problems with the rubber over barb connections, but I'd upgrade it to AN for the sake of reliability. To go overboard, you might as well hook up the coolant lines to the wastegate and turbo.
Turbo is based on preference, power goals, and budget. You can use the GT35 61mm (~620hp) or the 55mm (~550) and still make power, but it might only last 20k miles. They are cost-effective enough that you can replace them quite easily. But as I get older, I instead set it and don't have to worry about it. Hence why I would use a Garret BB or Borg Warner S300 series turbo. I know the S300 is not ball bearing, but they move air quite well, are cost-effective, and spool just as quickly as a ball bearing. The only thing I would worry about is the front turbo housing being too big. Also might need to make some customization to the downpipe.
Any other turbo kits I'd choose?
On3 also released a stage 2 version of their kit, in which it offers a T4 manifold compared to the open v-band. It will allow you to fit a plethora of different turbos and make power more efficiently than the v-band manifold. I still choose the v-band manifold for the ease of turbo install/uninstall. I find the extra ~100hp I'm missing out on isn't worth the convenience.
The speed tech N54 kit is also a kit I would consider, and it's priced well; I've installed quite a few of these kits, and they have held up well. The only thing I would change is the oil drain line. But all in all impressed with that kit. The only reason I haven't switched to the SpeedTech is its bottom mount. You'd don't have to worry about the heat that much with a bottom mount setup, but if I ever wanted to service the turbo, it takes more time than a top mount.