just to put a practical spin on things, and to widen your search no-end, is to look out for VW/Audi/Seat/ fit wheels, and some vauxhall 4 stud too,
they are all 4 x 100 if from the right era, such as Mk1/2 and some 3 Golfs, and can be fitted with 2mm wobble bolts no problem at all.
that should help you find something, however, 19's weren't really the thing in the Mk1/2 Golf era, they came a bit later, aswell as with the general move towards 5 stud for the extra size and weight of the big rims, or performance bias of a car that is designed to have 19" wheels, so again you might it a tad difficult.
adaptors of course can be used, and then you could fit all sorts, i'd go for a 5 x 100 pattern from your 4 x 98 original pattern, and then 19's will be a doddle to source/find, but youre going to be adding offset/width all the time too, so wide arches will become a neccssity i suspect?
but then, and now the personal view, 19's on a car of that era, i think i'd want wide arches to house them in and a big wide stance too.
i am a wheel hore, love messing with rims! but i think i'd draw the line at 17's with a 40, maybe 45 for comfort, profile, but that is my own personal view, so you go ahead and try whatever you want to try mate,
no harm in that is there.
finally, there comes a time where larger wheels start to do the opposite to what you want to achieve i find.
i always want the larger size to sit in the arch better, so i usualy mix a larger wheel, with a subtle lower/drop of the car itself, and that helps the car look properly modified, in my opinion,
but when the wheels are so big that they fill the arch on their own, you also start to lift the car because of the increase in total diameter/rolling radius, but because they fill the arch already, you can't lower the car any back towards terra firma,
so the wheels look awesome buried into the arch, cool, but the car ends up on stilts, not cool!
its a balance thing i find, and sometimes i get it right, and sometimes i don't,
go ahead and try it fella, it can't hurt if youre just buying a single wheel to try it out eh.