All antennas that are supposed to be soldered permanently to pieces of electronics are specified under certain conditions, say what size ground plane they have. Measurements of such antennas are useless in other contexts.
For example, a ceramic GPS antenna produced by Taoglas may be intended for mounting in the center of a piece of PCB and resonate at the right frequency when it is. But if its mounted at the end of a device where it doesn't have a ground plane not only wont it resonate at the right frequency, which means that it wont get the polarization right as its a highly optimized design that shrinks the size of a patch antenna and depends on all sorts of factors relating to whats around it. Off tune it will also likely function worse than a (spiral) piece of wire cut to the correct frequency and diameter and also wont have the proper polarization meaning rejection of signals that are not GPS. its likely.
Making the significant extra expenditure on a "real" GPS antenna useless. Installed badly, you fancy, small ceramic antenna may be less than useless. Buying a commercial antenna doesn't guarantee you anything except spending the money. Understand the users responsibility is to provide the antenna with the right environment so it can work. Make sure you interact in a meaningful way with the manufacturer to get the support you are paying for by buying their product. Thats absolutely essential if ceramic materials are involved. Because that tells you its a system, with missing parts, besides the antenna itself.
A good VNA can give you much of the information you will need to do this properly but not all of it. The other half is YOU. You need some knowledge of how RF behaves and some common sense, and some trial and error to get it consistently right. As you see, now you can spend a lot less. But if you get a functional VNA that is just the beginning of a learning journey. Its going to be a fairly similar journey no matter how much you spend on a VNA, if it is a VNA. Best that it be an inexpensive journey if you are a beginner. Maybe later on when you really NEED a better one for some specific reason and your expenditure is likely to be a needed one then spend more on one then.
A plastic case may be better in terms of accuracy.
The best advice I can give you is call them (the antenna manufacturer) on the phone and have a discussion with your manufacturers support person, and if you can send them a picture of your proposed usage of their antenna in situ, as it were. I would ask them what their antenna gives you that a monopole doesnt. There is a good chance that its simply a size reduction. It may have lower gain than a monopole.
Ask them to sell you their antenna. (Justify the expense to you). Just because a family of devices claims long range, doesn't mean that their devices actually possess the ability to traverse that long range. Given the nature of VHF and UHF, it may actually be impossible for the signals to go very far if the devices at the two ends of the link are no more than a meter or two above the ground.
Speak to them. Thats almost a must-do if real money is about to be spent. Better safe than sorry.