Moving from 185/65/15 to 205/45/17 should be fine, by my calculations that's only 0.8% smaller than the original wheels so when your speedo reads 70mph you will be doing 69.5mph (assuming your speedo is spot-on to start with).
Watch the offset*, though. Too small and it'll stick out of the arches, too high and it'll catch the suspension. I'd go for 40-42mm depending on the wheel.
I had 205/40/17s on my 1.5 and now my 1.8 - bloody rough ride at the best of times, big potholes scare the crap out of you and the wheels don't fill those big arches that well - so go for 205/45/17s.
About 50-quid a tyre for OK tyres - maybe half that for crappy budget tyres, twice that or more for really good ones.
Try and get tyres with a rim protector - no matter how hard you try you'll likely scuff a kerb at some point and those first scratches on pristine alloys really hurt the soul.
Another thing to remember is that bigger wheels often mean more unsprung weight (even when going from steelies to alloys) which adversely affects handling and acceleration. May not be that noticable but it's worth considering the weight of the new wheels as some are much lighter or heavier than others.
The N16 Almera is heavy for it's size so hills are it's enemy and the 1.5 is not that powerful to start with. Mine was fine up to 70 then started to flag.
Try and get locking nuts that weigh about the same as the standard nuts - no point getting your wheel/tyre nicely balanced then throwing it off with a heavy lump of metal in one quarter of the wheel.
That's all I can think of right now.
*
everything you wanted to know about wheels and tyres but were afraid to ask