If you want to go geared in the future, buy a wheelset to suit. You can run a single cog on the freehub with PVC pipe as spacers or even fit a cassette and just use one gear. Fit a derailleur as a chain tensioner. You can either buy a good one now or just any old cheapie/second hand/reject just to keep the chain under control - you're not trying to change gears so you only need the dr for the spring tension.

However, you will need both brakes and brake levers. The brakes can be used again but also come with a groupset. Brake levers will have to be discarded when you go to gears.

I personally reckon what you're proposing is a mug's game unless you're willing to buy your groupset in bits and pieces (not the most cost effective). Take a deep breath, do it properly now and save later on. You can also consider going to Tiagra rather than 105 - considering the development over the years, you're still getting a damned good groupset.

An alternative, if you don't mind different and your frame has mounts for cable adjustment, is to set her up as a SS now (as described in paragraph one), buying Tektro brake levers which are actually pretty good, then buying a set of Dura Ace bar end shifters. You can either mount them on the bar ends or on the frame in the traditional down tube position. Different and not 'racy' but cheaper than buying funny shifters.

I'd also keep my eye on places like Pro-bike kit and ChainReaction cycles, online shops that have a lot of different specials.

The crankset is irrelevant, just buy what you'll need later because it will do the job whatever you choose to do.

Generally speaking - it's dearer to build a bike than buy one, dearer to build one up then change it's type than do it right the first time and, most importantly, having a 'groupset' is irrelevant because over time, you'll change this bit, then that bit, then something else so any bike that serves you a long time will become a mixed bag of components.