The language is influenced by Latin, Church Slavonic and Middle High German, as well as Kipchak and Oghuz branches of Turkic; Hungarian and German being the main source of transmitted loanwords. Due to its isolation in the mountains, the language was able to preserve many archaic features; however monolingualism is commonplace since Middle Ages.
PS: there is no consensus about the origin of Szekelys, a Turkic hypothesis relates it to the Turkic tribal name Esegel/Eskil and the language and speaker's name can also be derived from it (Eskel, for the time being); meanwhile other peoples, including Magyars and Romanians, migrating into this area got assimilated to this language.
Now I find it difficult to build the vocabulary, while avoiding being too similar to Chuvash. There is a Turkic Swadesh list on Wiktionary, and over 1/3 of about 300 words in Chuvash cannot find cognate in any other Turkic languages (however, one should also aware of semantic shift). I have no idea whether they're of Uralic origin, or ancient roots preserved in Oghuric only, or from an unknown substrate in Volga-Ural area; I only make use of roots present in Starostin's database for the time.Among other writers who mentioned Esegels, the Persian “Geography“ of 982 named Ishkils as one of three Bulgarian tribes, who were constantly conflicting among themselves. Gardizi, the author of the composition Zain al-ahbar (mid-11th century), wrote: "Between possessions of Bulgars and possessions of Eskels, who also belong to Bulgars, is a Magyar area. These Magyars are also a Türkic tribe". Constantine Porphyrogenitus wrote that endoethnonym of the "Magyar Türks" was Savartoiaskaloi, i.e. Savart (Suvar/Sabir) and Eskel. Yu.A.Zuev summarized that It is held that Eskels (Esegels) merged with Hungarians (Magyars). The ethnographic group Sekei (written form Szekler) are recognized as their descendants. Esegels left a prominent trace in the Northern and Western Europe as last and first names, mostly in the form "Askel".
As for sound changes, I don't know the condition of the a>ɯ shift in Oghur (for short *a only?) and how to do with vowel length in closed and open syllables. Another difficulty is onomastics: placenames and personal names of Eskel area, as toponymy in Szeklerland *here* shows no sign of Turkic influence.
Besides I also want to create a descendent of Vlach language in Slovakia and Moravia; other projects include a group of (perhaps para-Chamic) Austronesian langs spoken in Mauritius, Seychelles and Reunion; a paleo-Balkanic lang spoken in Northeast Balkans.