wait, the tithe echelon? With a harvester, crawler, boneshaper, soulreaper, 3 stalkers/immortis, and 20 morteks? If you have two of those already, I wouldn't personally get a third, no. The chances of you needing more than two boneshapers or soulreapers are virtually nill, the chances of you needing more than two harvesters or crawlers are pretty low, and the stalkers & morteks aren't enough in themselves to be worth getting a third box.
IMO, wait for the 3e release, which will probably come with a fomo box and/or a vanguard set. are the battleforces all you currently have? If so, and you can't wait, you're better off trying to find the old broken realms box set (ten deathriders plus a liege at a significant discount), or if you can't find that then consider picking up katakros, arkhan, or even nagash, as they're cool models & relatively unlikely to be packaged in a discount vanguard box in the future. Not impossible - arkhan used to be in a start collecting and katakros's nighthaunt counterpart lady olynder has found her way into a couple of discount boxes - they're less likely to show up in a future discount box than stalkers, morteks, or deathriders..
OH! I almost forgot the nexus. If you don't already have the faction terrain 'bonetithe nexus' get that next. It's free to field so every OBR army wants one, it's also highly unlikely to end up in a discount set, plus the model is huge and kind of a pain to paint, so working on that will likely eat up a significant amount of time as you wait for the eventual battletome release.