I wouldn't go below a 5" screen and I'm not sure you can get a HB or Lowrance for that. Possibly $300 on sale. Call Brian at BBG Marine, a CDC sponsor. He's a great guy and will make you the best price.
However, you might want to look on here for a used unit. Most of us are equipment junkies and upgrade a unit that is just like new just to get a bigger unit.
The HB Helix5" combo GPS is a solid 1st unit.