I am gonna get roasted by some of the purists around here but I think the Antares autotune, used sparingly, is a great plug in. Yeah you can resing the part in tune. Yeah you can mark a file and use a pitch shifter. But for fast results which don't eat up 30 minutes of the day this is just a great plug.
That said, I use it very lightly so that it does not have the Shania Twain sound and it sounds natural.
Cakewalks built in limiter is pretty good. I use and really like the Ultrafunk Suite of plug-ins. They are top quality and sound great. It includes a 5 band Multiband, 6 point parametric EQ, Digital Delay, Phaser, Modulator, Gate, Reverb, Surround and Wah Wah effect.
If your getting up to $300 I would add a little more and get a UAD-1 card. Its dedicated hardware effects processor is one of the best values in homerecording. The limiter and compression on this card come very close to the sounds you will hear on the radio. Or, you can do this externally with and RNC and maybe a used dbx 160,
160X limiter.
You might want to invest in a tube or condensor mic to match with your DMP3. The Marshall V69, Studio Projects low end tube, or better yet you can get the NTK for like $385. All of these will get you closer to a top end vocal.
Sorry I'm getting out of your budget.....