first, you need a used machine. Its your only choice for something thats going to work on the beach. since you only have $400 I would look for a used minelab safari, fisher CZ3D, or fisher CZ20. You might be able to find one of those for around $400-500.
The minelab sov is another choice but I would be sure you can get it repaired if it were to break as I know they are not doing work on some of the sov models anymore since its been out of production now for several years. The minelab explorer is another option too but make sure you can get it repaired first as those are also out of production now as well.
The best beach machines are the Excalibur, CZ21, and CTX but they are way out of your price range ($1200-$2500 new). Unfortunately detecting on the beach means most expensive machines because they need to be multifrequency and waterproof. The ones I recommended to you are not waterproof but you don't have much of a choice so just dont drop them in the ocean.
If you wanted a PI machine that would work too but I would not recommend one to you unless you had a very good reason for getting it. Examples of PI include the tesaro sand shark and whites dual field. These will not be fun at your beaches unless you want to be digging up pieces of fish hooks and old iron scraps. You only use a PI on the NC beaches for very specific purposes.
Just remember if its not multifrequency its not going to work on the beach any better than your current machine. Do you research on the machines people recommend and make sure they will work where you live.
You also need a good scoop if you don't have one. The one I have, 920 stealth scoop starts at around $200 and goes up over $400 for a carbon handle. So yea, beach detecting gets expensive fast. But one gold ring or cell phone can net you several hundred dollars so if you put in the time and learn your area it can be worth...but never get into metal detecting for money.
