Praise be to Allah.
The suitor is obliged to tell his potential fiancée about any defect that may affect marital life, or affect his ability to fulfil the rights of his wife and children, or that his wife may find off-putting. Epilepsy is one of these defects, so he is obliged to disclose it and it is haraam to conceal it.
Ibn al-Qayyim (may Allah have mercy on him) said: By analogy, any defect that could put one spouse off the other, and lead to the objectives of marriage, namely compassion and affection, not being attained means that the other spouse must be given the choice (of continuing in the marriage or ending it by divorce).
End quote from Zaad al-Ma‘aad (5/166).
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: The correct view is that a defect is anything that may lead to the objectives of marriage not being attained, and undoubtedly the most important objectives of marriage include intimacy, looking after one another, and having children. If there is anything that could prevent these objectives being attained, then it is regarded as a defect. Based on that, if the wife finds out that the husband is infertile, or the husband finds that the wife is infertile, then it is a defect.
End quote from ash-Sharh al-Mumti‘ (12/220).
Shaykh Saalih al-Fawzaan (may Allah preserve him) was asked: My brother has epilepsy, but this will not prevent him from having intercourse. He has done the marriage contract with a woman; is he obliged to tell her about this before he consummates the marriage with her, or not?
He replied: Yes, either of the spouses must tell the other of any physical defects that they have before going ahead with marriage, because this is part of being sincere and honest, and because it is more likely to lead to harmony between them, and ward off disputes, so that each of them will make a commitment to the other on the basis of insight. It is not permissible to deceive or conceal.
End quote from al-Muntaqa min Fataawa al-Fawzaan.
To sum up: you are obliged to disclose this defect to the girl to whom you want to propose, so that she may agree to marry you on the basis of full knowledge, and so that you may avoid any deceit or concealment.
We ask Allah, may He be exalted, to grant you well-being and healing from every disease.
And Allah knows best.