I only recommended waiting because the TP isn't designed for you to be able to buy everything unless you're spending sacred crystals on charisma refills or ignoring special events religiously on Thursdays. Spica is a great unit though, as long as you keep her safe.

As far as the premium summons go, it really depends on what your priority is. It probably is a good idea to premium summon a few times when you are new to bolster your ranks with some (hopefully) good units. The units featured currently are all ones I'd consider good, aside from the Pegasus knight, which I would consider average since she's only a gold, and their stats are already lower than Valkyries.

If you're not in any rush to summon, though, I'd recommend waiting til you have 50 or 100 SC and then doing it all at once, since your monthly rewards card changes for every 50 SC spent. The deluxe card (100SC spent) gives 2 SC instead of 1 on certain days, which makes it the most appealing to me.