A good component for the money, so good in fact I know people that race on this stuff, is the Shimano 105 group. Their inexpensive and very reliable.

Some of the cheapest costing rims yet still very good are Sun rims.

Handlebars can be had cheaply as long as you don't go with some carbon fiber bar which would be crazy to do when you could spend that money instead on better wheels which is where you feel a noticeable performance difference vs lighter bars.

And piecing a bike together is more expensive then buying the whole group as a whole, BUT if you don't have the cash and your only way to buy a group is to put it on a credit card then you lost the savings anyways with paying high interest. So buying one component at a time as you can afford it is not a bad idea entirely. You can do the same thing with tools, as you buy a component and you need specialized tool(s) to do the job then you just buy that tool(s).