This can be an incredibly expensive part of any bathroom remodel, especially if you plan to move fixtures.
To keep the costs to a budget, keep the footprint of your bathroom the same. Otherwise, you might need to have more than plumbing works done as you may have to install new pipes and move electrical components.
However, to get the best out of your project; generally, the existing bathroom would need re-arranging.
As a general rule, we would prefer to install all pipework from new and the electrics in this area, so nothing becomes problematic later on from using the existing.
Cabinets, like floors, come in several price points. Pre-made cabinets are the least expensive, but they may not be what you want.
The big-box cabinets might not withstand the moisture in a bathroom, and you might have to replace them quicker than expected.
Consider the materials the cabinets are made of before you make your choice, especially knowing that the cabinets will get a serious amount of use in a tough environment.