EDIT: Was typing all this up when a horde of linguists posted before me. Oh well, I add whatever help I can to fighting the good fight.
nor that we should be sticking strictly to real history
But we should use it to check against absurdities. It is the only case-study we have.
but I doubt anyone will be capable of reaching a consensus when it comes to words.
A consensus is unnecessary when we are blessed with good suggestions and Symphony's ultimate decision
Besides, deciding these things universally seems historically a poor choice as, while many words in one language are based off those in another language, I cannot think of any languages that borrowed their entire set of basic words from another language,
Latin: ego, tu / me, te / nos, vos
Italian: io, tu / mi, ti / noi, voi
Spanish: yo, tu / me, te / nosotros, vosotros
French: je, tu / me, moi, to, toi / nous, vous
Latin: mater / pater
Greek: meiteir / pateir (its an eta, not epsilon-iota, but I'm being lazy)
Latin: cornu (horn) / tyrannus / magna / prius / cum- / quinque
Greek: cera (horn) / tyrannos / mega / protos / sun- / pente
(Note: Quinque and pente are from the same IE root. For some reason, apparently Latin words tend toward q's while Greek tends towards p's, t's, and d's). While some can be chalked up to later borrowing, I am going to have to disagree with you, if I understand you right. An examination of Sanskrit alongside Greek and Latin would show many similarities in roots traceable back to the so-called Indo-European language. Some of these words aren't even basic, like horn or tyrant.
If you are starting with a relatively small group of peoples, it is likely their language will be very similar, only diverging after long periods of separation (not complete separation - see Romance languages) or contact with other languages.
Also, Romance languages are all, more or less, degenerate* versions of Latin, and so borrow almost everything from it. (*not morally... well, maybe not morally

)
If I'm misunderstanding something, let me know, but I don't think coming up with some root words is a bad idea at all. Of course, these can and will eventually be diverged from to the point of unrecognizability, just not yet.
One final suggestion: don't let "to be" have a regular conjugation. I haven't yet seen one that is.
Random Samplings of "to be" verbs
Greek: eimi, ei, esti, esmen, este, eisi, ein, eistha, ein...
Latin: esse, sum, es, est, sumus, estis, sunt
English: be, is, are, was, were
Spanish: ser, soy, eres, es, somos, esteis, son, estar