Reliable? Yes very. 170k miles, tuned since 120k and I am not super soft on it. ( automatic as well) just change the oil at 5k religiously.
expensive? well that depends on how much you are willing to do yourself. at 70k miles it shouldn't need too much done to it... relatively. at 70k you are approaching the need for a new timing belt and tensioners. The recommended is 80k or so I believe. This can be done on your own relatively easily( easy process, 8/10 frustrating job due to space) if you have the tools and know how. A shop will want 800-900$ but that is for the belts, tensioners and water pump. My water pump finally caught a case of "crunchy bearing" at 160k but I am in the south with low rust/ salt issues. engine mounts, trans mount. will need replacing when they break, average is around 100-120k I think.
around 120-140 you will begin to have to start replacing a good number of things that are simply at the end of their life. examples: radiator hoses, coolant tank, brake lines, TCV, etc. everything can be done at home as there are plenty of guides. the cost for doing it yourself will be substantially less but you may have to purchase tools.
the expensive part of owning a volvo isn't necessarily the parts,( they are more expensive than say honda or toyota but not terrible), it's the shop hourly rate as most places charge "European car" hourly rate.
note: prices below are based on item costs and not including tool, misc or labor costs that a shop would charge.
Items that I had to replace or make changes due to age or issues: headlight lenses, had to remove, clean and put reflective aluminum tape on the reflector as the chrome coating wears away with time. $20~ put HID bulbs and ballast in 100~ rear sway bar mounts $50~ front sway bar rod/mount things 50-100~ rear hatch handle: (snapped off in sub zero temps and hatch froze closed. ) ~60-100 engine and trans mounts: 200-300 for all. TCV/CBV (unsure cost)
around 150k my suspension struts finally died but was safe and drive-able yet bouncy until replacement. (cost varies between aftermarket, performance or Volvo OEM).
there are a number of youtube videos that list "what to look out for when buying a p1 volvo", one from mynameideasweretaken is very well done.
Hope this helps.