You don't want a PA to be flat or to sound good. That's not its job. The role of a PA is to make it so you can hear the vocals and other quiet instruments over all of the other instruments that are already loud. If stage monitors had a flat frequency response, they would be terrible for the job. When I set up stage monitors, I actually cut out a lot of low frequencies because they are just going to suck up power without making the vocals easier to hear.
Unfortunately, you're right about cost. Like everything else in audio, the better stuff almost always costs more. And to power monitors that can really take your head off with vocals in front of a fairly loud band you probably want at least 500 Watts per channel. And feedback is always going to be a concern.
Your near field monitors are not going to cut it. I'd be surprised if they were even that audible. And yes, you may be tempted to crank them so loud that you damage them.
You can start with a single powered PA speaker to act as a monitor, you don't have to buy a huge system. The larger the band, the more speakers you potentially want to make sure everyone can hear. More speakers spread out and set to lower volume is better than a few speakers turned up really loud because you want to control reflections and dispersion to help prevent feedback. At the same time, how you arrange the speakers can create comb filtering which can actually encourage feedback.
If you invest in a pair of decent quality powered PA speakers, you will almost always have a good use for them. Even though they are usually marketed to be used as floor monitors, and they can do a decent job as floor monitors, PA speakers are never the best floor monitors. That's because PA speakers should have fairly wide dispersion and floor monitors should be very very tight.
For some reason the only speakers I've found that are designed to be dedicated floor monitors are all unpowered (except some very expensive ones). So I have two unpowered wedges, an amp that pushes about 500 watts per channel, and I also have an automatic feedback suppression unit with two channels. Many sound engineers hate automatic feedback suppression, but for the monitors I find it to be so valuable. I literally never worry about feedback.
Regarding feedback, one of the best things you can do to prevent it is to reduce reflections inside the practice space. Actual acoustic treatment is most effective at this, but putting rugs down on any hard floors and hanging drapes everywhere will help in a pinch. Large plate windows will be your worst enemy unless you can hang drapes in front of them.
Building a good monitor system is not easy. If you are gigging, it would be a good idea try get a business card or contact info from any mix engineers that you like, and perhaps ask them if they would come by and give you any advice.