Our new puppy is 8 weeks old and has some, but not all, rounds of her vaccinations. Before she came home we bought a used plastic-based travel crate from a second-hand store. We've cleaned it out with 409, but after learning more about parvo, etc. we have not used the travel crate yet. Today we have a lot of errands to run and the travel crate would be perfect to use. Is there a risk of disease transmission from the used crate?
Yes, there is a risk of disease from the crate; no, that doesn't mean you can't use it.
409 is not sufficient to kill parvovirus. The only common cleaner that reliably kills parvo is bleach, so you should clean the crate thoroughly with a solution of bleach before using it with a puppy. But bleach is quite effective against this virus, and if you get it good and clean, it's as safe as a brand new one (maybe safer, there's no guarantee that a new crate hasn't managed to get parvo in it somehow!)
You should also consider the risks that your puppy is already exposed to. Parvo is very common in the environment, because wild animals can carry the canine parvovirus and leave it wherever they go. If your puppy goes outside regularly, it's probably already being exposed. This is why many breeders are cautious when allowing prospective adopters to meet puppies - the humans can easily bring parvovirus in with them, even if they don't even have a dog.
Puppy vaccination series are often referred to as "booster" shots, a term which is not really appropriate in this case because each vaccine either works or it doesn't. There's no magic number of vaccines that a puppy must receive in order to achieve immunity, and the vaccines don't build on each other. They are all basically the same vaccine, and the reason multiple shots are given is because it's hard to know which vaccination is the one that is going to "take."
Puppies receive antibodies from their mother's milk, and the presence of these antibodies will protect the puppy but will also block a vaccine from taking effect. As a puppy is weaned, this protection fades. It's important to give a vaccine after the maternal antibodies have cleared out, but before the puppy has a chance to become infected. The puppy is at risk during this window of time because the period it can become infected is the same as the period when a vaccine can work.
This is why veterinarians sometimes advise keeping puppies isolated from the world before their vaccinations have been completed. Unfortunately, this is also a critical time in the social and mental development of puppies and isolation can lead to them becoming fearful or otherwise poorly-adjusted. Your best bet is to work with your vet to find places where your puppy can go while being as safe from infection as possible. Carrying your puppy will help keep her safe. Puppy kindergarten is a good idea as well.
To sum up, wash out your crate with bleach, and go ahead and use it. Once you've thoroughly cleaned it, it's probably safer than taking the dog for a walk.
(most of this information comes from this book - I am not a veterinarian. Although the book is breed-specific, the sections on health and nutrition are generally applicable).
Is there a risk of disease transmission from the used crate? Yes.
There is always a risk, however large or small, of disease transmission, injury, and so on from basically anything you'll ever do with your dog. Whether that risk is 0.0000001% or 99.997% depends on a slew of information which you haven't included (and probably don't know). Knowing what the risk is, rather than merely the existence of risk in general, will help you make thoughtful decisions about your pup.
For context, the typical rate of infection for Parvo is less than 1% in most areas, excluding periods of time when there is an outbreak. The rate of infection for Lepto is higher (around 8%, barring an outbreak and depending on the area you're in).
You can clean the crate with bleach, although you'll need to wait before putting your pup in there since it will smell quite bad and bleach is dangerous if it gets on your dogs skin. This will kill most of the germs in the crate, but nothing is 100% effective.