In my location, I use transplants for squash & other cucurbits not because of pests (the gardens are completely fenced) but because of temperamental weather. If the soil temperature and/or moisture is less than optimal (a frequent occurrence) direct-seeding has spotty results, sometimes no germination at all. Even in a good Spring, germination would usually be slow, up to 2 weeks. Seed started indoors germinates much more quickly, and at a much higher percentage.
As others have already stated, squash seedlings grow quickly - so this is not one of the vegetables I would recommend starting early to extend the season. Two weeks before the anticipated transplanting date is a good starting point. For best results, the transplants should go in when the seedlings have their first true leaf. I've put them in as late as 3 weeks after planting; but much beyond that, they will become root bound & stunted.
For best results, the transplants should be started in something the roots can grow through (peat, paper, soil blocks) and transplanted with as little root damage as possible. I use peat strip pots, and place about 1/2" of sand in the tray beneath the pots. This helps to keep the pots moist, and allows the roots to wander without being air pruned. Those roots can be carefully pulled from the sand at transplanting time, and if handled carefully & kept moist, will help the plants recover quickly.
Don't have any observations about sunflowers, I never tried them as transplants. There are a couple bird feeders near my rural garden, so there are always 6-10 volunteers, and unless they are in the way, I just plant around them.
