Important parts of a kit, other than the actual generator, are pretty standard. You will need mounting brackets and hardware, tubing or hose to run the gas to it's destination, and supplies to wire the unit for electricity. Electrical components should consist of wires, fuses, possibly a switch. You will need a fitting or some such to attach the hose to the engine. Hose clamps might be a part of the package.
It isn't always easy to tell if a product is built correctly until you have the unit in your hands. One way to find out is to check the item description by clicking on the picture, then seeing if an other custoers have left feedback for a similar item. Other than that, you can compare features of differnt units and you might be able to spot problems with a particular part, etc.
Does the seller have a guarrantee, or a money back return period if you want to send the generator back to him? This is a good sign that the creator is confident in his unit and will stand behind it 100%. You can email the seller, if possible, and see how the communication is before you buy. This doesn't mean it wul be the sam after the sale, but still it might be an indicator of the seller's intentions and such.
It is difficult to estimate if a particular hydrogen generator kit will fit in your vehicle. You need to check the item's description and look for measurements. Then, go to the vehicle you intend to attach it to with a tpe measure andtry to visualize how it will fit in there. Sometimes it is immedately apparent that there is no way it will work. On other vehicles there will be room and to spare.
As far as whether or not a particular hho fuel cell will work for you, again your best bet is to look for testimonials or positive feedback from previous customers of this unit. Other than that, if there is a moneyback guarrantee you might just have to take and chance and go for the plunge. An easy rule of thumb is, if it sounds way too good to be true, proceed with caution.
As you can see, there are a wide variety og hydrogen generator kits out there, so you may have to do a little research to find a good one that will fit your needs. Check here for do it yourself hho hydrogen information. Most of the descriptions have some good info as well, so click on a picture of an item to get the full story.