Suntour was the BEST stuff ever make, I have a bike with 100% Superbe on it and it has over 160,000 miles on the components and not a single repair, and about 50,000 of that was racing, but about 150,000 of it was a combination of daily mountain riding and some flat.
Anyway, you can't get that stuff new so on to the discussion.
Not sure what kind of terrain you'll be riding, but Tiagra has a wide gear range it can use, so if you need gears for steep climbs and you're not as strong of a rider, having a wider gear range could be helpful. Tiagra most people believe is better than Veloce.
With Centaur vs 105, the gear ratio choice is a bit larger with the Centaur, but 105 has a snappier shift response, and better reliability. The other consideration is that 105 replacement parts are less expensive, and easier to maintain.
If you are on a budget I would go with Shimano, and due to the greater reliability, less weight, better shifting, I would go with 105 and not Tiagra.
You could do a bit of a mix if you want slightly better shifting, go with all 105, but get a Ultegra rear derailleur, it shouldn't cost more than $40 extra, but I'm not sure if Ripple will do a swap, if not, then just wait till the 105 fails which will be about 15 years down the road if you're averaging 5,000 miles a year, then replace it with Ultegra. On my Lynskey I have 105 components except the rear derailleur is Ultegra, that bike is 12 years old with roughly 40,000 miles and not a single repair, but maintenance is the key to making them last.