IMHO quick change tool posts on small machines are not, perhaps, as satisfactory as might ideally be hoped. Although all engineering is a compromise, the art being to get the performance you need at a price you can afford with the minimal number of work arounds where real life collides with infinite desire, small QC systems on small lathes can leave the feeling that everything in your favour is against you. Of necessity QC tool support is indirect, round the corner, and via extra sliding joints when compared to a bolt down type. Given that the objective is to present the tool rigidly and accurately to the work the QC system inevitably compromises tool mounting stiffness which is acceptable providing there is enough stiffness left. With a small machine all the components, both in toolpost and lathe are disproportionately weaker than with a lager machine soley by reason of their small size. Affordablity in relation to amateur pockets is a very important consideration inevitably ruling out sophisticated design and close tolerance manufacture which might otherwise help overcome the limitations of small size.
Given that you have already decided that a QC set-up is within budget if a suitable one can be found its worth seeing if there is a way of getting most of the benifits of a QC set-up without loosing the stiffness inherent in bolt down systems. In the past I've found an economy "fairly fast" change toolpost system based on interchanging 4 way tool posts to be adequately satisfactory so maybe a refined version would work for you.
In practice you rarely manage to get more than two tools in the small four ways so I'd suggest 4 will be needed. Cutting up beer cans to shim up on the machine is, as you have found, unsatisfactory. It should be fairly easy to devise something to let you set the tools to centre height on the bench, preferably at a constant projection. A modest investment in known size shim stock, I'm told that the colour coded pastic type works well, and sheet metal spacers, worn out and de toothed hacksaw blades can be handy, should make the whole setting process pretty painless. Especially if you can spare a dial gauge or other measuring device so as to know exactly how much shim to add.
Best to fit a rear toolpost mount whilst you are at it then the parting tool and a simple flat top round nose 45° point facing / chamfering tool can live there. Satisfactiory basic loads for the other post would seem to be :- roughing tool & finishing tool, boring tools large & small, threading tools inside and outside. Depending on the machine and detail 4-way design you may need some low cunning to get really satisfactory indexing for the 180° shifts needed on the front tools and the 90° one on the back but nothing seems unreasonable. Obviously you may need to change the specific tools for different materials or for tricky work. I recommend that you grind the angles on the tool rather than pivot the holder to make one tool do several jobs.
When sharp and well supported the simple flat top facing tool works remarkably well. It may be easier to make a U section carrier into which the facing tool and its necessary shims can be clamped via suitable grub screws so the rear four way doesn't have to be disturbed after sharpening that tool. If you want to stick with one four way such U section carriers could be made for all the tooling if arranged so that the could be swopped in and out of the four way without altering the projection. If using simple HSS bar for tooling it would be advantageous to make the U with a built in up kick as used on Armstrong style holders so height can be set simply by varaying the projection. The amount of fuss and effort needed to produce the necessary quantity of U sections is deceptive and a considerable effort. Multiple 4 ways is much easier.
Clive