IME, a horse that is even somewhat broke should be safe to ride almost as soon as you pull them off the trailer. If they aren't, then there is some holes in the training that would need to be addressed.
The whole "let them settle in" thing never really made much sense to me. Horses that get hauled all over the continent for rodeos or shows or trail rides don't get a week or more to "settle" to the new surroundings before being expected to behave under saddle. Why should the expectation be any less for any other horse in the world? Anyway, sorry for my off topic rant LOL.
Even my customer horses that I got in for training, I would frequently be riding them either the same day they arrived or the day after...and these were often horses that had not been ridden.
Personally, if it was me and I could afford it, I'd go ahead and get a PPE and include x-rays of the legs and hooves. A good vet should be able to tell you whether the lameness is actually due to an abscess or whether there may be something more serious lurking in there too.