Quite a challenging request I think, but since nobody has provided comments so far, my 2 cents, fwiw are
1. Building a bike from components because shop-bought ones cost a lot is (usually) contradictory in terms. Complete bikes are generally significantly cheaper than the sum of component parts, even if tools and time to build were free. If a bike with specific features one wants is not available the economical solution is probably to get the closest thing and swap the unwanted parts out. Building from scratch is usually worthwhile only if one wants to learn and appreciates the fun of doing it, it is most unlikely to be the economical solution.
2. I have never tried it but I have some doubt about the value of combining a shock-absorbing saddle and (I presume you meant) suspension seatpost, given each has its inherent challenges. Further a set of spring on top of another is no different to a single spring with a long stroke, which will not help to maintain your saddle height and therefore your pedalling effectiveness and efficiency. On the other hand, a good set of balloon tyres with supple walls (such as the Schwalbe Big Apple, which is specifically designed to offer a plush ride) is probably the most cost effective and efficient suspension money can buy.
3. If an off-the-shelf bike is to be the basis and it must have hub gears, it will limit the choice to a handful (e.g. Specialized Globe, On-One Pompetamine, Trek Soho, Giant Seek 0 etc.), but perhaps even fewer if at all if it must be able to accommodate fat (say 2" wide) tyres for decent shock absorption. Afaik no hub-geared bike is currently sold with rear suspension.
4. Indeed if ride plushness is of paramount importance and money is no object, imho the best option is just to get a quality full suspension bike - a couple are lighter than 10 kgs these days, albeit at a price. If not, you can do worse than seeking out the best/lightest hybrid/rigid mtb/hardtail that can accommodate fat tyres you can afford, and install the fattest Big Apple it can take.
5. If handlebar position and posture are important, it is worth starting off with an adjustable stem.
6. The other option of course is to consider a recumbent, but that is a subject even further away from my knowledge and experience...
