Offset is the horizontal distance the hub face of the wheel is away from the centre line of the wheel, expressed in millimeters.
Zero offset would mean that the hub face of the wheel is directly in line with the centre line of the wheel (you can imagine such a wheel would stick out of the arch a fair bit).
A 40mm offset means the hub face of the wheel is 40mm short of the centre line (viewed from outside the wheel) so that wheel would fit "into" the wheel arch more. The bigger the offset the more "into" the arch the wheel goes.
If you fit spacers for example, they effectively force the wheel to be further "out" than they would otherwise be, so they mimic reducing the offset.
The offset is shown on the wheel by a number with the letters "ET" (e.g. ET35 or ET38 etc.)
"Wobble" bolts are wheel bolts with a floating cup washer/shoulder. If you fit 100 PCD wheels to your hubs (98 PCD) each bolt shoulder would not fit "flush" against the wheel bolt-holes since the holes on the wheel are 100mm apart (measured through the centre) while the hub holes are 98mm apart. Only the inner shoulder of the bolt would be pressing against the wheel bolt hole.
The floating cup allows the shoulder to seat square in the hole. Once tight the bolts don't "wobble".. it's just the cup that has play in it (when loose) compared to a regular 1-piece wheel bolt.
You can get wobble bolts from most wheel retailers... they were invented so people could fit wheels with slightly different PCD to their hubs..,.. (Fiat/Alfa being a classic one since they have a pretty unique 98mm PCD while most of Europe (VW Opel etc.) use a 100mm PCD.. so there are way more wheels are made in 100mm PCD than 98mm (which is pretty unique to Fiat/Alfa/Lancia.. and Lada I believe.. a throw-back to the old Fiat 124/125 models they re-branded in the late 1960's and continued to make ever since..
If you use the regular black/anthracite Speedlines, then you don't need to worry about any of this, since they will fit the 155 (and were actually made for it). The tyre you want for that is a 195/50 x 16 or 215/45x16. I'm not sure what was fitted as OE but these are ball-park the same size as a 205/40x17 so should fit.