No, the Plus test isn't for infant seats. Only seats up to 18kg, 25kg, 105cm or 125cm.
There was another test they did recently in Norway which tested a load of seats in a very similar set up to the plus test, but for infant seats. But everything they tested is over OP's budget.
The thing is if you're going for a 25kg seat after the infant seat, then you don't need to worry about centiles. If you have a high centile child, then you'll need the 25kg seat, and if you have a low centile child, happy days, you can rear face them until they're 8 or 9!
Spin seats are not really easier to get children in or out of than 25kg seats. 25kg seats have low, open sides so they are easy to maneuvre into and out of. Spin seats can be more of a pain especially once the child is over about 18 months because they hit their head on the ceiling, and this is after we moved to an SUV... Children over 2 can often climb into 25kg seats themselves, some even younger, so then you only have to strap them in.
OTOH, if you think you'll have more than one child, it can work out to get a spin seat for straight after the infant carrier and then a 25kg seat for afterwards, when the spin seat is needed by DC2. Then they just move through them in turn.
Centiles can go all over the place in the first year, especially if you have any issues with feeding, but yeah you should have an idea by 3-4 months or so. Just a note, centiles are not taken from birth weight any more but from weight at 2 weeks old or after baby has regained birth weight. This is because centiles can change quite dramatically from birth and this isn't usually relevant - birth weight is affected by gestation, size of mother, and gestational diabetes among other things, and may not be indicative of future centiles.
I did include a couple of non i-size seats in the list :) Only the ones with good side impact protection. I agree that in a city, the side impact is quite important, because a side impact is more likely in city roads with more T-junctions, crossroads etc. Whereas if you're doing more motorway driving, the frontal impact score is probably more important. All the seats I mentioned have bases available and all of the bases are isofix. TBH there are not very many non-isofix bases.