I personally think that the best thing you can do to get yourself ready to write as a genuine beginning-of-the-17st-century person is to read the first edition of El ingenioso hidalgo don Quijote de la Mancha, first published in 1605. Just the original title gives you a hint of what you can expect:
El ingenioso hidalgo don Qvixote de la Mancha.
Yes, it uses X as the nowadays J. And it uses V as the nowadays U and vice versa (example: nouenta marauedis) as they were considered variations of the same letter. Just have a look at the first pages (Tassa and El rey) and you will discover more:
- The use of the Ç as the nowadays Z (examples: conde de Benalcaçar, onças, lança).
- The use of the double S in some words (examples: lector carissimo, sossiego).
- The use of the ~ not only in the Ñ to make abbreviations, usually omitting a following N (examples: se hace menciõ, lleno de pẽsamiẽtos varios).
- The use of Y in some cases instead of I (examples: seyscientos, traygais, vereys).
- The use of the ZE and ZI syllables instead of CE, CI (examples: hazerlos, dezia).
- The lack of some H's (examples: ay instead of hay, aora instead of ahora, although agora was also used in this case).
Apart from that, a lot of different words were used. You can check the Covarrubias dictionary (1611) to see if some word was of use in the first years of the XVII century. Tip: go to the NTLLE, look up a word and see if an entry titled "Covarrubias" appear.
As for the grammar, yes, the future subjuntive was way more used than today:
[...] y la otra tercia parte, para el juez que lo sentenciare.
- The vos (second person plural) pronoun was used instead of the nowadays usted.
- The second person plural termination of verbs was still sometime -des instead of -eis, but that form was disappearing at the time: huuieredes, hallaredes, trataredes.
- Pronouns suffixed to verbs: llenosele instead of se le llenó.
- Lacks almost all the nowadays tildes.
I could carry on and on, but I really think you should check the book for yourself and then start asking questions about the words, forms and constructions you do not understand, one at a time. That would get you the best documentation for your book and will bring very interesting questions about the history of the Spanish language to this site.