Software issues are a Mk4 Octavia issue, the Mk3 & Mk3.5 are generally okay AFAIK.
The infotainment system is much, much nicer IMO. Bigger, clearer, capacitative rather than resistive. Very nice! I do like the "proper" buttons of the old system, but all things considered, I prefer the new.
The looks you get used to. I've now grown to love my Mk3.5's distinctive look, but I can understand why a lot of people don't like it.
Additionally, the 1.5 TSI is far more likely than the 1.4 to have a particulate filter fitted. One more thing to go wrong, but generally the 1.5 TSI seems to be a pretty good engine.
One plus of the 1.5 is that it's likely to be mated to a later gearbox, and the later the DSG the better.
I know everyone seems to like the 1.0 engine, but if buying 2nd hand, I personally wouldn't touch one if I was buying a car to keep for some years. I think it's a bit underpowered, but then I like big grunty engines having driven diesels for a couple of decades.
Not sure if the 150ps engines have IRS, but I'm pretty sure the 115ps engines don't have it.
I think your idea of a 1.4/1.5 petrol DSG SE L would be a good call, especially if you can find one spec'd with one or two nice extras like heated seats or the full winter pack.
I wouldn't worry too much about the silica bag issue. Check the coolant is nice and pink and the heating inside the car is even on driver and passenger side, then whip that silica bag out and replace the G13 coolant with G12evo and you should be golden.